Get run_test_cases.py to fail if it fails to get the list of tests it should run.

tools/isolate/run_test_cases.py

Index: tools/isolate/run_test_cases.py
diff --git a/tools/isolate/run_test_cases.py b/tools/isolate/run_test_cases.py
index 323204877a8646148d432bafce539b9af1c32a63..9beccc80696924811c95888a5ee1d1f54de57692 100755
--- a/tools/isolate/run_test_cases.py
+++ b/tools/isolate/run_test_cases.py
@@ -707,7 +707,9 @@ def main(argv):
         options.shards)
 
   if not test_cases:
-    return 0
+    # If test_cases is None then there was a problem generating the tests to
+    # run, so this should be considered a failure.
+    return int(test_cases is None)
 
   if options.no_dump:
     result_file = None