Do not consider missing command as internal failure.

client/tests/run_isolated_smoke_test.py

 #!/usr/bin/env python
 # Copyright 2012 The Swarming Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0 that
 # can be found in the LICENSE file.
 
 import json
 import logging
 import os
 import subprocess
 import sys
@@ skipping 277 matching lines @@
     self.assertEqual('Success\n', out, out)
     self.assertEqual(0, returncode)
     actual = list_files_tree(self.cache)
     self.assertEqual(sorted(set(expected)), actual)
 
   def test_fail_empty_isolated(self):
     isolated_hash = self._store_isolated({})
     expected = ['state.json', isolated_hash]
     out, err, returncode = self._run(self._cmd_args(isolated_hash))
     self.assertEqual('', out)
-    self.assertIn('No command to run\n', err)
+    self.assertIn(
+        '<The .isolated doesn\'t declare any command to run!>\n'
+        '<Check your .isolate for missing \'command\' variable>\n',
+        err)
     self.assertEqual(1, returncode)
     actual = list_files_tree(self.cache)
     self.assertEqual(sorted(expected), actual)
 
   def test_includes(self):
     # Loads an .isolated that includes another one.
 
     # References manifest2.isolated and repeated_files.isolated. Maps file3.txt
     # as file2.txt.
     isolated_hash = self._store('check_files.isolated')
@@ skipping 70 matching lines @@
     cached_file_path = self._test_corruption_common(
         CONTENTS['file1.txt'][:-1] + ' ')
     # TODO(maruel): This corruption is NOT detected.
     # This needs to be fixed.
     self.assertNotEqual(CONTENTS['file1.txt'], read_content(cached_file_path))
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   test_utils.main()