Re-raise exceptions instead of sys.exit

Test: th
Change-Id: I3ac343b51eeeaa948712be81b640122f9981137d
diff --git a/tools/releasetools/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index 3db5559..22c6ac4 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -1383,6 +1383,6 @@
     main(sys.argv[1:])
   except common.ExternalError as e:
     print("\n   ERROR: %s\n" % (e,))
-    sys.exit(1)
+    raise
   finally:
     common.Cleanup()