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

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Runs each test cases as a single shard, single process execution.
 
 Similar to sharding_supervisor.py but finer grained. Runs multiple instances in
 parallel.
 """
@@ skipping 689 matching lines @@
       test_cases = filter(None, f.read().splitlines())
   else:
     test_cases = get_test_cases(
         executable,
         options.whitelist,
         options.blacklist,
         options.index,
         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
   else:
     if options.result:
       result_file = options.result
     else:
       result_file = '%s.run_test_cases' % executable
 
   return run_test_cases(
       executable,
       test_cases,
       options.jobs,
       options.timeout,
       result_file)
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))