diff --git a/input_tools/android/common/src/main/java/com/android/libcore/timezone/util/Errors.java b/input_tools/android/common/src/main/java/com/android/libcore/timezone/util/Errors.java
index 24c608c..e1e48f9 100644
--- a/input_tools/android/common/src/main/java/com/android/libcore/timezone/util/Errors.java
+++ b/input_tools/android/common/src/main/java/com/android/libcore/timezone/util/Errors.java
@@ -16,12 +16,17 @@
 
 package com.android.libcore.timezone.util;
 
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.io.StringWriter;
 import java.util.ArrayList;
 import java.util.LinkedList;
 import java.util.List;
 
 /**
- * Stores context, errors and error severity for logging and flow control.
+ * Stores context, errors and error severity for logging and flow control. This class distinguishes
+ * between warnings (just info), errors (may not be immediately fatal) and fatal (immediately
+ * fatal).
  */
 public final class Errors {
 
@@ -45,19 +50,29 @@
         return scopes.removeLast();
     }
 
-    public void addFatal(String msg) {
-        level = Math.max(level, LEVEL_FATAL);
-        add(msg);
+    /** Adds a fatal error, and immediately throws {@link HaltExecutionException}. */
+    public HaltExecutionException addFatalAndHalt(String msg) throws HaltExecutionException {
+        addInternal(msg, null, LEVEL_FATAL);
+        throw new HaltExecutionException("Fatal error");
+    }
+
+    /** Adds a fatal error, and immediately throws {@link HaltExecutionException}. */
+    public HaltExecutionException addFatalAndHalt(String msg, Throwable t)
+            throws HaltExecutionException {
+        addInternal(msg, t, LEVEL_FATAL);
+        throw new HaltExecutionException("Fatal error");
     }
 
     public void addError(String msg) {
-        level = Math.max(level, LEVEL_ERROR);
-        add(msg);
+        addInternal(msg, null, LEVEL_ERROR);
+    }
+
+    public void addError(String msg, Throwable t) {
+        addInternal(msg, t, LEVEL_ERROR);
     }
 
     public void addWarning(String msg) {
-        level = Math.max(level, LEVEL_WARNING);
-        add(msg);
+        addInternal(msg, null, LEVEL_WARNING);
     }
 
     public String asString() {
@@ -73,15 +88,46 @@
         return messages.isEmpty();
     }
 
+    /** True if there are error or fatal messages. */
     public boolean hasError() {
         return level >= LEVEL_ERROR;
     }
 
+    /** True if there are fatal messages. */
     public boolean hasFatal() {
         return level >= LEVEL_FATAL;
     }
 
-    private void add(String msg) {
+    private void addInternal(String msg, Throwable t, int level) {
+        this.level = Math.max(this.level, level);
+        addMessage(msg);
+        if (t != null) {
+            try (StringWriter out = new StringWriter();
+                    PrintWriter printWriter = new PrintWriter(out)) {
+                t.printStackTrace(printWriter);
+                addMessage(out.toString());
+            } catch (IOException e) {
+                // Impossible - this is actually a compiler bug. Nothing throws IOException above.
+                throw new AssertionError("Impossible exception thrown", e);
+            }
+        }
+    }
+
+    private void addMessage(String msg) {
         messages.add(scopes.toString() + ": " + msg);
     }
+
+    /** Throws a {@link HaltExecutionException} if there are any error or fatal messages. */
+    public void throwIfError(String why) throws HaltExecutionException {
+        if (hasError()) {
+            throw new HaltExecutionException(why);
+        }
+    }
+
+    /** Thrown to halt execution. */
+    public static class HaltExecutionException extends Exception {
+        HaltExecutionException(String why) {
+            super(why);
+        }
+    }
 }
diff --git a/input_tools/android/common/src/test/java/com/android/libcore/timezone/util/ErrorsTest.java b/input_tools/android/common/src/test/java/com/android/libcore/timezone/util/ErrorsTest.java
index 5a6e717..1d568e5 100644
--- a/input_tools/android/common/src/test/java/com/android/libcore/timezone/util/ErrorsTest.java
+++ b/input_tools/android/common/src/test/java/com/android/libcore/timezone/util/ErrorsTest.java
@@ -20,8 +20,10 @@
 
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
 
 import com.android.libcore.timezone.testing.TestUtils;
+import com.android.libcore.timezone.util.Errors.HaltExecutionException;
 
 public class ErrorsTest {
 
@@ -62,7 +64,10 @@
         assertFalse(errors.hasError());
         assertFalse(errors.hasFatal());
 
-        errors.addFatal("Hello");
+        try {
+            throw errors.addFatalAndHalt("Hello");
+        } catch (HaltExecutionException expected) {}
+
         assertFalse(errors.isEmpty());
         assertTrue(errors.hasError());
         assertTrue(errors.hasFatal());
@@ -80,13 +85,19 @@
         errors.addError("John Cleese");
 
         errors.pushScope("Holy grail");
-        errors.addFatal("Silly place");
+        try {
+            errors.addFatalAndHalt("Silly place");
+            fail();
+        } catch (HaltExecutionException expected) {}
         errors.popScope();
 
         errors.addError("Michael Palin");
 
         errors.pushScope("Parrot sketch");
-        errors.addFatal("Fjords");
+        try {
+            errors.addFatalAndHalt("Fjords");
+            fail();
+        } catch (HaltExecutionException expected) {}
         errors.popScope();
 
         String[] lines = errors.asString().split("\n");
diff --git a/input_tools/android/telephonylookup_generator/src/main/java/com/android/libcore/timezone/telephonylookup/TelephonyLookupGenerator.java b/input_tools/android/telephonylookup_generator/src/main/java/com/android/libcore/timezone/telephonylookup/TelephonyLookupGenerator.java
index 978d264..4626d8d 100644
--- a/input_tools/android/telephonylookup_generator/src/main/java/com/android/libcore/timezone/telephonylookup/TelephonyLookupGenerator.java
+++ b/input_tools/android/telephonylookup_generator/src/main/java/com/android/libcore/timezone/telephonylookup/TelephonyLookupGenerator.java
@@ -19,6 +19,7 @@
 
 import com.android.libcore.timezone.telephonylookup.proto.TelephonyLookupProtoFile;
 import com.android.libcore.timezone.util.Errors;
+import com.android.libcore.timezone.util.Errors.HaltExecutionException;
 
 import com.ibm.icu.util.ULocale;
 
@@ -68,78 +69,75 @@
     }
 
     boolean execute() throws IOException {
-        // Parse the countryzones input file.
-        TelephonyLookupProtoFile.TelephonyLookup telephonyLookupIn;
+        Errors errors = new Errors();
         try {
-            telephonyLookupIn = parseTelephonyLookupTextFile(telephonyLookupProtoFile);
-        } catch (ParseException e) {
-            logError("Unable to parse " + telephonyLookupProtoFile, e);
-            return false;
-        }
+            // Parse the countryzones input file.
+            TelephonyLookupProtoFile.TelephonyLookup telephonyLookupIn;
+            try {
+                telephonyLookupIn = parseTelephonyLookupTextFile(telephonyLookupProtoFile);
+            } catch (ParseException e) {
+                throw errors.addFatalAndHalt("Unable to parse " + telephonyLookupProtoFile, e);
+            }
 
-        List<TelephonyLookupProtoFile.Network> networksIn = telephonyLookupIn.getNetworksList();
+            List<TelephonyLookupProtoFile.Network> networksIn = telephonyLookupIn.getNetworksList();
 
-        Errors processingErrors = new Errors();
-        processingErrors.pushScope("Validation");
-        validateNetworks(networksIn, processingErrors);
-        processingErrors.popScope();
+            validateNetworks(networksIn, errors);
+            errors.throwIfError("One or more validation errors encountered");
 
-        // Validation failed, so stop.
-        if (processingErrors.hasFatal()) {
-            logInfo("Issues:\n" + processingErrors.asString());
-            return false;
-        }
-
-        TelephonyLookupXmlFile.TelephonyLookup telephonyLookupOut =
-                createOutputTelephonyLookup(networksIn);
-        if (!processingErrors.hasError()) {
-            // Write the output structure if there wasn't an error.
+            TelephonyLookupXmlFile.TelephonyLookup telephonyLookupOut =
+                    createOutputTelephonyLookup(networksIn);
             logInfo("Writing " + outputFile);
             try {
                 TelephonyLookupXmlFile.write(telephonyLookupOut, outputFile);
             } catch (XMLStreamException e) {
-                e.printStackTrace(System.err);
-                processingErrors.addFatal("Unable to write output file");
+                throw errors.addFatalAndHalt("Unable to write output file", e);
+            }
+        } catch (HaltExecutionException e) {
+            e.printStackTrace();
+            logError("Stopping due to fatal error: " + e.getMessage());
+        } finally {
+            // Report all warnings / errors
+            if (!errors.isEmpty()) {
+                logInfo("Issues:\n" + errors.asString());
             }
         }
-
-        // Report all warnings / errors
-        if (!processingErrors.isEmpty()) {
-            logInfo("Issues:\n" + processingErrors.asString());
-        }
-
-        return !processingErrors.hasError();
+        return !errors.hasError();
     }
 
-    private static void validateNetworks(
-            List<TelephonyLookupProtoFile.Network> networksIn, Errors processingErrors) {
-        Set<String> knownIsoCountries = getLowerCaseCountryIsoCodes();
-        Set<String> mccMncSet = new HashSet<>();
-        for (TelephonyLookupProtoFile.Network networkIn : networksIn) {
-            String mcc = networkIn.getMcc();
-            if (mcc.length() != 3 || !isAsciiNumeric(mcc)) {
-                processingErrors.addFatal("mcc=" + mcc + " must have 3 decimal digits");
-            }
+    private static void validateNetworks(List<TelephonyLookupProtoFile.Network> networksIn,
+            Errors errors) {
+        errors.pushScope("validateNetworks");
+        try {
+            Set<String> knownIsoCountries = getLowerCaseCountryIsoCodes();
+            Set<String> mccMncSet = new HashSet<>();
+            for (TelephonyLookupProtoFile.Network networkIn : networksIn) {
+                String mcc = networkIn.getMcc();
+                if (mcc.length() != 3 || !isAsciiNumeric(mcc)) {
+                    errors.addError("mcc=" + mcc + " must have 3 decimal digits");
+                }
 
-            String mnc = networkIn.getMnc();
-            if (!(mnc.length() == 2 || mnc.length() == 3) || !isAsciiNumeric(mnc)) {
-                processingErrors.addFatal("mnc=" + mnc + " must have 2 or 3 decimal digits");
-            }
+                String mnc = networkIn.getMnc();
+                if (!(mnc.length() == 2 || mnc.length() == 3) || !isAsciiNumeric(mnc)) {
+                    errors.addError("mnc=" + mnc + " must have 2 or 3 decimal digits");
+                }
 
-            String mccMnc = "" + mcc + mnc;
-            if (!mccMncSet.add(mccMnc)) {
-                processingErrors.addFatal("Duplicate entry for mcc=" + mcc + ", mnc=" + mnc);
-            }
+                String mccMnc = "" + mcc + mnc;
+                if (!mccMncSet.add(mccMnc)) {
+                    errors.addError("Duplicate entry for mcc=" + mcc + ", mnc=" + mnc);
+                }
 
-            String countryIsoCode = networkIn.getCountryIsoCode();
-            String countryIsoCodeLower = countryIsoCode.toLowerCase(Locale.ROOT);
-            if (!countryIsoCodeLower.equals(countryIsoCode)) {
-                processingErrors.addFatal("Country code not lower case: " + countryIsoCode);
-            }
+                String countryIsoCode = networkIn.getCountryIsoCode();
+                String countryIsoCodeLower = countryIsoCode.toLowerCase(Locale.ROOT);
+                if (!countryIsoCodeLower.equals(countryIsoCode)) {
+                    errors.addError("Country code not lower case: " + countryIsoCode);
+                }
 
-            if (!knownIsoCountries.contains(countryIsoCodeLower)) {
-                processingErrors.addFatal("Country code not known: " + countryIsoCode);
+                if (!knownIsoCountries.contains(countryIsoCodeLower)) {
+                    errors.addError("Country code not known: " + countryIsoCode);
+                }
             }
+        } finally {
+            errors.popScope();
         }
     }
 
diff --git a/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/TzLookupGenerator.java b/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/TzLookupGenerator.java
index 95525bf..f1175f0 100644
--- a/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/TzLookupGenerator.java
+++ b/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/TzLookupGenerator.java
@@ -19,6 +19,7 @@
 import com.android.libcore.timezone.tzlookup.zonetree.CountryZoneTree;
 import com.android.libcore.timezone.tzlookup.zonetree.CountryZoneUsage;
 import com.android.libcore.timezone.util.Errors;
+import com.android.libcore.timezone.util.Errors.HaltExecutionException;
 import com.ibm.icu.util.BasicTimeZone;
 import com.ibm.icu.util.Calendar;
 import com.ibm.icu.util.GregorianCalendar;
@@ -74,11 +75,6 @@
 
     /**
      * Executes the generator.
-     *
-     * Positional arguments:
-     * 1: The countryzones.txt file
-     * 2: the zone.tab file
-     * 3: the file to generate
      */
     public static void main(String[] args) throws Exception {
         if (args.length != 3) {
@@ -97,85 +93,110 @@
         this.outputFile = outputFile;
     }
 
-    boolean execute() throws IOException {
-        // Parse the countryzones input file.
-        CountryZonesFile.CountryZones countryZonesIn;
+    boolean execute() {
+        Errors errors = new Errors();
         try {
-            countryZonesIn = CountryZonesFileSupport.parseCountryZonesTextFile(countryZonesFile);
-        } catch (ParseException e) {
-            logError("Unable to parse " + countryZonesFile, e);
-            return false;
-        }
+            // Parse the countryzones input file.
+            CountryZonesFile.CountryZones countryZonesIn =
+                    parseAndValidateCountryZones(countryZonesFile, errors);
 
-        // Check the countryzones rules version matches the version that ICU is using.
-        String icuTzDataVersion = TimeZone.getTZDataVersion();
-        String inputIanaVersion = countryZonesIn.getIanaVersion();
-        if (!icuTzDataVersion.equals(inputIanaVersion)) {
-            logError("Input data (countryzones.txt) is for " + inputIanaVersion
-                    + " but the ICU you have is for " + icuTzDataVersion);
-            return false;
-        }
+            // Check the countryzones.txt rules version matches the version that ICU is using.
+            String icuTzDataVersion = TimeZone.getTZDataVersion();
+            String inputIanaVersion = countryZonesIn.getIanaVersion();
+            if (!icuTzDataVersion.equals(inputIanaVersion)) {
+                throw errors.addFatalAndHalt("Input data (countryzones.txt) is for "
+                        + inputIanaVersion + " but the ICU you have is for " + icuTzDataVersion);
+            }
 
-        // Pull out information we want to validate against from zone.tab (which we have to assume
-        // matches the ICU version since it doesn't contain its own version info).
-        ZoneTabFile zoneTabIn = ZoneTabFile.parse(zoneTabFile);
-        Map<String, List<String>> zoneTabMapping =
-                ZoneTabFile.createCountryToOlsonIdsMap(zoneTabIn);
-        List<CountryZonesFile.Country> countriesIn = countryZonesIn.getCountriesList();
-        List<String> countriesInIsos = CountryZonesFileSupport.extractIsoCodes(countriesIn);
 
-        // Sanity check the countryzones file only contains lower-case country codes. The output
-        // file uses them and the on-device code assumes lower case.
-        if (!Utils.allLowerCaseAscii(countriesInIsos)) {
-            logError("Non-lowercase country ISO codes found in: " + countriesInIsos);
-            return false;
-        }
-        // Sanity check the countryzones file doesn't contain duplicate country entries.
-        if (!Utils.allUnique(countriesInIsos)) {
-            logError("Duplicate input country entries found: " + countriesInIsos);
-            return false;
-        }
+            // Pull out information we want to validate against from zone.tab (which we have to
+            // assume matches the ICU version since it doesn't contain its own version info).
+            Map<String, List<String>> zoneTabMapping = parseZoneTabFile(zoneTabFile, errors);
 
-        // Validate the country iso codes found in the countryzones against those in zone.tab.
-        // zone.tab uses upper case, countryzones uses lower case.
-        List<String> upperCaseCountriesInIsos = Utils.toUpperCase(countriesInIsos);
-        Set<String> timezonesCountryIsos = new HashSet<>(upperCaseCountriesInIsos);
-        Set<String> zoneTabCountryIsos = zoneTabMapping.keySet();
-        if (!zoneTabCountryIsos.equals(timezonesCountryIsos)) {
-            logError(zoneTabFile + " contains "
-                    + Utils.subtract(zoneTabCountryIsos, timezonesCountryIsos)
-                    + " not present in countryzones, "
-                    + countryZonesFile + " contains "
-                    + Utils.subtract(timezonesCountryIsos, zoneTabCountryIsos)
-                    + " not present in zonetab.");
-            return false;
-        }
+            List<CountryZonesFile.Country> countriesIn = countryZonesIn.getCountriesList();
+            List<String> countriesInIsos = CountryZonesFileSupport.extractIsoCodes(countriesIn);
 
-        Errors processingErrors = new Errors();
-        TzLookupFile.TimeZones timeZonesOut = createOutputTimeZones(
-                inputIanaVersion, zoneTabMapping, countriesIn, processingErrors);
-        if (!processingErrors.hasError()) {
+            // Sanity check the countryzones file only contains lower-case country codes. The output
+            // file uses them and the on-device code assumes lower case.
+            if (!Utils.allLowerCaseAscii(countriesInIsos)) {
+                throw errors.addFatalAndHalt(
+                        "Non-lowercase country ISO codes found in: " + countriesInIsos);
+            }
+            // Sanity check the countryzones file doesn't contain duplicate country entries.
+            if (!Utils.allUnique(countriesInIsos)) {
+                throw errors.addFatalAndHalt(
+                        "Duplicate input country entries found: " + countriesInIsos);
+            }
+
+            // Validate the country iso codes found in the countryzones.txt against those in
+            // zone.tab. zone.tab uses upper case, countryzones uses lower case.
+            List<String> upperCaseCountriesInIsos = Utils.toUpperCase(countriesInIsos);
+            Set<String> timezonesCountryIsos = new HashSet<>(upperCaseCountriesInIsos);
+            Set<String> zoneTabCountryIsos = zoneTabMapping.keySet();
+            if (!zoneTabCountryIsos.equals(timezonesCountryIsos)) {
+                throw errors.addFatalAndHalt(zoneTabFile + " contains "
+                        + Utils.subtract(zoneTabCountryIsos, timezonesCountryIsos)
+                        + " not present in countryzones, "
+                        + countryZonesFile + " contains "
+                        + Utils.subtract(timezonesCountryIsos, zoneTabCountryIsos)
+                        + " not present in zonetab.");
+            }
+
+            TzLookupFile.TimeZones timeZonesOut = createOutputTimeZones(
+                    inputIanaVersion, zoneTabMapping, countriesIn, errors);
+            errors.throwIfError("Errors accumulated");
+
             // Write the output structure if there wasn't an error.
             logInfo("Writing " + outputFile);
             try {
                 TzLookupFile.write(timeZonesOut, outputFile);
             } catch (XMLStreamException e) {
-                e.printStackTrace(System.err);
-                processingErrors.addFatal("Unable to write output file");
+                throw errors.addFatalAndHalt("Unable to write output file", e);
+            }
+            return true;
+        } catch (HaltExecutionException | IOException e) {
+            logError("Stopping due to fatal condition", e);
+            return false;
+        } finally {
+            // Report all warnings / errors
+            if (!errors.isEmpty()) {
+                logInfo("Issues:\n" + errors.asString());
             }
         }
+    }
 
-        // Report all warnings / errors
-        if (!processingErrors.isEmpty()) {
-            logInfo("Issues:\n" + processingErrors.asString());
+    private Map<String, List<String>> parseZoneTabFile(String zoneTabFile, Errors errors)
+            throws HaltExecutionException {
+        errors.pushScope("Parsing " + zoneTabFile);
+        try {
+            ZoneTabFile zoneTabIn;
+            zoneTabIn = ZoneTabFile.parse(zoneTabFile);
+            return ZoneTabFile.createCountryToOlsonIdsMap(zoneTabIn);
+        } catch (ParseException | IOException e) {
+            throw errors.addFatalAndHalt("Unable to parse " + zoneTabFile, e);
+        } finally {
+            errors.popScope();
         }
+    }
 
-        return !processingErrors.hasError();
+    private static CountryZonesFile.CountryZones parseAndValidateCountryZones(
+            String countryZonesFile, Errors errors) throws HaltExecutionException {
+        errors.pushScope("Parsing " + countryZonesFile);
+        try {
+            CountryZonesFile.CountryZones countryZonesIn;
+            countryZonesIn = CountryZonesFileSupport.parseCountryZonesTextFile(countryZonesFile);
+            return countryZonesIn;
+        } catch (ParseException | IOException e) {
+            throw errors.addFatalAndHalt("Unable to parse " + countryZonesFile, e);
+        } finally {
+            errors.popScope();
+        }
     }
 
     private static TzLookupFile.TimeZones createOutputTimeZones(String inputIanaVersion,
             Map<String, List<String>> zoneTabMapping, List<CountryZonesFile.Country> countriesIn,
-            Errors processingErrors) {
+            Errors errors) throws HaltExecutionException {
+
         // Start constructing the output structure.
         TzLookupFile.TimeZones timeZonesOut = new TzLookupFile.TimeZones(inputIanaVersion);
         TzLookupFile.CountryZones countryZonesOut = new TzLookupFile.CountryZones();
@@ -194,63 +215,60 @@
             String isoCode = countryIn.getIsoCode();
             List<String> zoneTabCountryTimeZoneIds = zoneTabMapping.get(isoCode.toUpperCase());
             if (zoneTabCountryTimeZoneIds == null) {
-                processingErrors.addError("Country=" + isoCode + " missing from zone.tab");
+                errors.addError("Country=" + isoCode + " missing from zone.tab");
                 // No point in continuing.
                 continue;
             }
 
             TzLookupFile.Country countryOut = processCountry(
                     offsetSampleTimeMillis, everUseUtcStartTimeMillis, countryIn,
-                    zoneTabCountryTimeZoneIds, processingErrors);
-            if (processingErrors.hasFatal()) {
-                // Stop if there's a fatal error, continue processing countries if there are just
-                // errors.
-                break;
-            } else if (countryOut == null) {
+                    zoneTabCountryTimeZoneIds, errors);
+            if (countryOut == null) {
+                // Continue processing countries if there are only errors.
                 continue;
             }
             countryZonesOut.addCountry(countryOut);
         }
+        errors.throwIfError("One or more countries failed");
         return timeZonesOut;
     }
 
     private static TzLookupFile.Country processCountry(long offsetSampleTimeMillis,
             long everUseUtcStartTimeMillis, CountryZonesFile.Country countryIn,
-            List<String> zoneTabCountryTimeZoneIds,
-            Errors processingErrors) {
+            List<String> zoneTabCountryTimeZoneIds, Errors errors) {
         String isoCode = countryIn.getIsoCode();
-        processingErrors.pushScope("country=" + isoCode);
+        errors.pushScope("country=" + isoCode);
         try {
             // Each Country must have >= 1 time zone.
             List<CountryZonesFile.TimeZoneMapping> timeZonesIn =
                     countryIn.getTimeZoneMappingsList();
             if (timeZonesIn.isEmpty()) {
-                processingErrors.addError("No time zones");
+                errors.addError("No time zones");
                 // No point in continuing.
                 return null;
             }
 
-            // Look for duplicate time zone IDs.
             List<String> countryTimeZoneIds = CountryZonesFileSupport.extractIds(timeZonesIn);
+
+            // Look for duplicate time zone IDs.
             if (!Utils.allUnique(countryTimeZoneIds)) {
-                processingErrors.addError("country's zones=" + countryTimeZoneIds
-                        + " contains duplicates");
+                errors.addError("country's zones=" + countryTimeZoneIds + " contains duplicates");
                 // No point in continuing.
                 return null;
             }
 
             // Each Country needs a default time zone ID (but we can guess in some cases).
-            String defaultTimeZoneId = determineCountryDefaultZoneId(countryIn, processingErrors);
+            String defaultTimeZoneId = determineCountryDefaultZoneId(countryIn, errors);
             if (defaultTimeZoneId == null) {
                 // No point in continuing.
                 return null;
             }
             boolean defaultTimeZoneBoost =
-                    determineCountryDefaultTimeZoneBoost(countryIn, processingErrors);
+                    determineCountryDefaultTimeZoneBoost(countryIn, errors);
 
             // Validate the default.
             if (!countryTimeZoneIds.contains(defaultTimeZoneId)) {
-                processingErrors.addError("defaultTimeZoneId=" + defaultTimeZoneId
+                errors.addError("defaultTimeZoneId=" + defaultTimeZoneId
                         + " is not one of the country's zones=" + countryTimeZoneIds);
                 // No point in continuing.
                 return null;
@@ -258,52 +276,47 @@
 
             // Validate the other zone IDs.
             try {
-                processingErrors.pushScope("validate country zone ids");
-                boolean errors = false;
+                errors.pushScope("validate country zone ids");
                 for (String countryTimeZoneId : countryTimeZoneIds) {
                     if (invalidTimeZoneId(countryTimeZoneId)) {
-                        processingErrors.addError("countryTimeZoneId=" + countryTimeZoneId
+                        errors.addError("countryTimeZoneId=" + countryTimeZoneId
                                 + " is not a valid zone ID");
-                        errors = true;
                     }
                 }
-                if (errors) {
+                if (errors.hasError()) {
                     // No point in continuing.
                     return null;
                 }
             } finally {
-                processingErrors.popScope();
+                errors.popScope();
             }
 
             // Work out the hint for whether the country uses a zero offset from UTC.
             boolean everUsesUtc = anyZonesUseUtc(countryTimeZoneIds, everUseUtcStartTimeMillis);
 
             // Validate the country information against the equivalent information in zone.tab.
-            processingErrors.pushScope("zone.tab comparison");
+            errors.pushScope("zone.tab comparison");
             try {
                 // Look for unexpected duplicate time zone IDs in zone.tab
                 if (!Utils.allUnique(zoneTabCountryTimeZoneIds)) {
-                    processingErrors.addError(
-                            "Duplicate time zone IDs found:" + zoneTabCountryTimeZoneIds);
+                    errors.addError("Duplicate time zone IDs found:" + zoneTabCountryTimeZoneIds);
                     // No point in continuing.
                     return null;
-
                 }
 
                 if (!Utils.setEquals(zoneTabCountryTimeZoneIds, countryTimeZoneIds)) {
-                    processingErrors.addError("IANA lists " + isoCode
+                    errors.addError("IANA lists " + isoCode
                             + " as having zones: " + zoneTabCountryTimeZoneIds
                             + ", but countryzones has " + countryTimeZoneIds);
                     // No point in continuing.
                     return null;
                 }
             } finally {
-                processingErrors.popScope();
+                errors.popScope();
             }
 
             // Calculate countryZoneUsage.
-            CountryZoneUsage countryZoneUsage =
-                    calculateCountryZoneUsage(countryIn, processingErrors);
+            CountryZoneUsage countryZoneUsage = calculateCountryZoneUsage(countryIn, errors);
             if (countryZoneUsage == null) {
                 // No point in continuing with this country.
                 return null;
@@ -315,20 +328,18 @@
 
             // Process each input time zone.
             for (CountryZonesFile.TimeZoneMapping timeZoneIn : timeZonesIn) {
-                processingErrors.pushScope(
+                errors.pushScope(
                         "id=" + timeZoneIn.getId() + ", offset=" + timeZoneIn.getUtcOffset()
                                 + ", shownInPicker=" + timeZoneIn.getShownInPicker());
                 try {
                     // Validate the offset information in countryIn.
-                    validateNonDstOffset(offsetSampleTimeMillis, countryIn, timeZoneIn,
-                            processingErrors);
+                    validateNonDstOffset(offsetSampleTimeMillis, countryIn, timeZoneIn, errors);
 
                     String timeZoneInId = timeZoneIn.getId();
                     boolean shownInPicker = timeZoneIn.getShownInPicker();
                     if (!countryZoneUsage.hasEntry(timeZoneInId)) {
                         // This implies a programming error.
-                        processingErrors.addFatal(
-                                "No entry in CountryZoneUsage for " + timeZoneInId);
+                        errors.addError("No entry in CountryZoneUsage for " + timeZoneInId);
                         return null;
                     }
 
@@ -343,13 +354,13 @@
                                     timeZoneInId, shownInPicker, notUsedAfterInstant);
                     countryOut.addTimeZoneIdentifier(timeZoneIdOut);
                 } finally {
-                    processingErrors.popScope();
+                    errors.popScope();
                 }
             }
             return countryOut;
         } finally{
             // End of country processing.
-            processingErrors.popScope();
+            errors.popScope();
         }
     }
 
@@ -357,20 +368,20 @@
      * Determines the default zone ID for the country.
      */
     private static String determineCountryDefaultZoneId(
-            CountryZonesFile.Country countryIn, Errors processingErrorsOut) {
+            CountryZonesFile.Country countryIn, Errors errors) {
         List<CountryZonesFile.TimeZoneMapping> timeZonesIn = countryIn.getTimeZoneMappingsList();
         String defaultTimeZoneId;
         if (countryIn.hasDefaultTimeZoneId()) {
             defaultTimeZoneId = countryIn.getDefaultTimeZoneId();
             if (invalidTimeZoneId(defaultTimeZoneId)) {
-                processingErrorsOut.addError(
+                errors.addError(
                         "Default time zone ID " + defaultTimeZoneId + " is not valid");
                 // No point in continuing.
                 return null;
             }
         } else {
             if (timeZonesIn.size() > 1) {
-                processingErrorsOut.addError(
+                errors.addError(
                         "To pick a default time zone there must be a single offset group");
                 // No point in continuing.
                 return null;
@@ -384,14 +395,14 @@
      * Determines the defaultTimeZoneBoost value for the country.
      */
     private static boolean determineCountryDefaultTimeZoneBoost(
-            CountryZonesFile.Country countryIn, Errors processingErrorsOut) {
+            CountryZonesFile.Country countryIn, Errors errors) {
         if (!countryIn.hasDefaultTimeZoneBoost()) {
             return false;
         }
 
         boolean defaultTimeZoneBoost = countryIn.getDefaultTimeZoneBoost();
         if (!countryIn.hasDefaultTimeZoneId() && defaultTimeZoneBoost) {
-            processingErrorsOut.addError(
+            errors.addError(
                     "defaultTimeZoneBoost is specified but defaultTimeZoneId is not explicit");
         }
 
@@ -459,7 +470,7 @@
         try {
             utcOffsetMillis = Utils.parseUtcOffsetToMillis(utcOffsetString);
         } catch (ParseException e) {
-            errors.addFatal("Bad offset string: " + utcOffsetString);
+            errors.addError("Bad offset string: " + utcOffsetString);
             return;
         }
 
@@ -471,7 +482,7 @@
 
         String timeZoneIdIn = timeZoneIn.getId();
         if (invalidTimeZoneId(timeZoneIdIn)) {
-            errors.addFatal("Time zone ID=" + timeZoneIdIn + " is not valid");
+            errors.addError("Time zone ID=" + timeZoneIdIn + " is not valid");
             return;
         }
 
@@ -481,7 +492,7 @@
         timeZone.getOffset(offsetSampleTimeMillis, false /* local */, offsets);
         int actualOffsetMillis = offsets[0];
         if (actualOffsetMillis != utcOffsetMillis) {
-            errors.addFatal("Offset mismatch: You will want to confirm the ordering for "
+            errors.addError("Offset mismatch: You will want to confirm the ordering for "
                     + country.getIsoCode() + " still makes sense. Raw offset for "
                     + timeZoneIdIn + " is " + Utils.toUtcOffsetString(actualOffsetMillis)
                     + " and not " + Utils.toUtcOffsetString(utcOffsetMillis)
@@ -490,21 +501,20 @@
     }
 
     private static CountryZoneUsage calculateCountryZoneUsage(
-            CountryZonesFile.Country countryIn, Errors processingErrors) {
-        processingErrors.pushScope("Building zone tree");
+            CountryZonesFile.Country countryIn, Errors errors) {
+        errors.pushScope("Building zone tree");
         try {
             CountryZoneTree countryZoneTree = CountryZoneTree.create(
                     countryIn, ZONE_USAGE_CALCS_START, ZONE_USAGE_CALCS_END);
             List<String> countryIssues = countryZoneTree.validateNoPriorityClashes();
             if (!countryIssues.isEmpty()) {
-                processingErrors
-                        .addError("Issues validating country zone trees. Adjust priorities:");
-                countryIssues.forEach(processingErrors::addError);
+                errors.addError("Issues validating country zone trees. Adjust priorities:");
+                countryIssues.forEach(errors::addError);
                 return null;
             }
             return countryZoneTree.calculateCountryZoneUsage(ZONE_USAGE_NOT_AFTER_CUT_OFF);
         } finally {
-            processingErrors.popScope();
+            errors.popScope();
         }
     }
 
diff --git a/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/ZoneTabFile.java b/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/ZoneTabFile.java
index a939ca5..6b960f9 100644
--- a/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/ZoneTabFile.java
+++ b/input_tools/android/tzlookup_generator/src/main/java/com/android/libcore/timezone/tzlookup/ZoneTabFile.java
@@ -19,6 +19,7 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Paths;
+import java.text.ParseException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
@@ -36,7 +37,7 @@
 
     private ZoneTabFile() {}
 
-    static ZoneTabFile parse(String zoneTabFile) throws IOException {
+    static ZoneTabFile parse(String zoneTabFile) throws IOException, ParseException {
         ZoneTabFile zoneTab = new ZoneTabFile();
 
         List<String> lines = Files
@@ -49,9 +50,9 @@
                         .collect(Collectors.toList());
 
         for (String mappingLine : mappingLines) {
-            String[] fields = mappingLine.split("\t");
+            String[] fields = mappingLine.split("\t+");
             if (fields.length < 3) {
-                throw new IOException("Line is malformed: " + mappingLine);
+                throw new ParseException("Line is malformed: " + mappingLine, 0);
             }
             CountryEntry countryEntry = new CountryEntry(fields[0], fields[2]);
             zoneTab.addCountryEntry(countryEntry);
@@ -74,7 +75,10 @@
                     countryEntry.isoCode, k -> new ArrayList<>());
             olsonIds.add(countryEntry.olsonId);
         }
-        return countryIsoToOlsonIdsMap;
+        // Replace each list value with an immutable one.
+        countryIsoToOlsonIdsMap.forEach(
+                (k, v) -> countryIsoToOlsonIdsMap.put(k, Collections.unmodifiableList(v)));
+        return Collections.unmodifiableMap(countryIsoToOlsonIdsMap);
     }
 
     static class CountryEntry {
diff --git a/input_tools/android/tzlookup_generator/src/test/java/com/android/libcore/timezone/tzlookup/ZoneTabFileTest.java b/input_tools/android/tzlookup_generator/src/test/java/com/android/libcore/timezone/tzlookup/ZoneTabFileTest.java
index b581ffd..108b902 100644
--- a/input_tools/android/tzlookup_generator/src/test/java/com/android/libcore/timezone/tzlookup/ZoneTabFileTest.java
+++ b/input_tools/android/tzlookup_generator/src/test/java/com/android/libcore/timezone/tzlookup/ZoneTabFileTest.java
@@ -25,6 +25,7 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.text.ParseException;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
@@ -32,7 +33,6 @@
 
 import static junit.framework.TestCase.assertEquals;
 import static org.junit.Assert.assertTrue;
-import static org.junit.Assert.fail;
 
 public class ZoneTabFileTest {
 
@@ -75,7 +75,7 @@
                 "# This is a comment",
                 "GB\tStuff\tEurope/London\tStuff",
                 "# This is a comment",
-                "US\tStuff\tAmerica/New_York\tStuff",
+                "US\tStuff\t\tAmerica/New_York\tStuff",
                 "# This is a comment",
                 "US\tStuff\tAmerica/Los_Angeles",
                 "# This is a comment"
@@ -89,14 +89,11 @@
                 zoneTab.getCountryEntries());
     }
 
-    @Test
+    @Test(expected = ParseException.class)
     public void parseMalformedFile() throws Exception {
         // Mapping lines are expected to have at least three tab-separated columns.
         String file = createFile("GB\tStuff");
-        try {
-            ZoneTabFile.parse(file);
-            fail();
-        } catch (IOException expected) {}
+        ZoneTabFile.parse(file);
     }
 
     @Test