Add an actionable error message when recipe_simulation_test.py fails

scripts/slave/unittests/recipe_simulation_test.py

 #!/usr/bin/env python
 # Copyright (c) 2013 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.
 
 import os
+import pipes
 import shutil
+import subprocess
 import sys
 
 import test_env  # pylint: disable=W0403,W0611
 
 # Delete the old recipe_engine directory which might have stale pyc files
 # that will mess us up.
 shutil.rmtree(os.path.join(
         os.path.dirname(os.path.dirname(os.path.dirname(os.path.dirname(
             os.path.realpath(__file__))))),
         'third_party', 'recipe_engine'),
     ignore_errors=True)
 
 RECIPES_PY = os.path.join(
     os.path.dirname(os.path.dirname(os.path.realpath(__file__))),
     'recipes.py')
 
-args = [sys.argv[0], 'simulation_test'] + sys.argv[1:]
-os.execvp(RECIPES_PY, args)
+args = [RECIPES_PY, 'simulation_test'] + sys.argv[1:]
+ret = subprocess.call(args)
+if ret:
+  # TODO(martiniss): Move this logic into recipes-py. http://crbug.com/601662
+  if not any(x.startswith('train') for x in args):
+    print
+    print 'To train new expectations, run:'
+    print '   ', sys.argv[0], 'train'
+    print
+  if not any(x.startswith('--html_report') for x in args):
+    print 'To create a coverage report, run:'
+    print '   ', ' '.join(pipes.quote(arg) for arg in sys.argv),
+    print '--html_report PATH'
+    print
+sys.exit(ret)