Refine assertion by providing more information. When the given path is NOT a
directory, output its path.

RELNOTES: n/a
PiperOrigin-RevId: 156187835
GitOrigin-RevId: 734d9e5663d86f9802c74a2ebd0e3ad9def0678e
Change-Id: I4c38374b1e2ee86994496aa9fdd34786ca56034e
diff --git a/java/com/google/devtools/build/android/desugar/LambdaClassMaker.java b/java/com/google/devtools/build/android/desugar/LambdaClassMaker.java
index 155e323..beaeb03 100644
--- a/java/com/google/devtools/build/android/desugar/LambdaClassMaker.java
+++ b/java/com/google/devtools/build/android/desugar/LambdaClassMaker.java
@@ -35,7 +35,8 @@
   private final Map<Path, LambdaInfo> generatedClasses = new LinkedHashMap<>();
 
   public LambdaClassMaker(Path rootDirectory) {
-    checkArgument(Files.isDirectory(rootDirectory));
+    checkArgument(
+        Files.isDirectory(rootDirectory), "The argument '%s' is not a directory.", rootDirectory);
     this.rootDirectory = rootDirectory;
   }