[lint] Fix lint to not raise an exception at the end.

recipe_engine/lint_test.py

 #!/usr/bin/env python
 # Copyright 2015 The LUCI 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.
 
 """Tests that recipes are on their best behavior.
 
 Checks that recipes only import modules from a whitelist.  Imports are
 generally not safe in recipes if they depend on the platform, since
 e.g. you can run a recipe simulation for a Windows recipe on Linux.
 """
 
 # TODO(luqui): Implement lint for recipe modules also.
 
 from __future__ import absolute_import
 import re
 import types
 
 
 MODULES_WHITELIST = [
+  r'ast',
   r'base64',
   r'collections',
   r'contextlib',
   r'copy',
   r'datetime',
   r'functools',
   r'hashlib',
   r'itertools',
   r'json',
   r'math',
   r're',
   r'urlparse',
   r'zlib',
 ]
 
 
-class ImportViolationError(Exception):
-  pass
-
-
-class TestFailure(Exception):
-  pass
-
-
 def ImportsTest(recipe_path, recipe_name, whitelist, universe_view):
   """Tests that recipe_name only uses allowed imports.
 
   Returns a list of errors, or an empty list if there are no errors (duh).
   """
 
   recipe = universe_view.load_recipe(recipe_name)
   for _, val in sorted(recipe.globals.iteritems()):
     if isinstance(val, types.ModuleType):
       module_name = val.__name__
@@ skipping 27 matching lines @@
   universe_view = loader.UniverseView(universe, package_deps.root_package)
 
   whitelist = map(re.compile, MODULES_WHITELIST + args.whitelist)
 
   errors = []
   for recipe_path, recipe_name in universe_view.loop_over_recipes():
     errors.extend(
         ImportsTest(recipe_path, recipe_name, whitelist, universe_view))
 
   if errors:
-    raise TestFailure('\n'.join(map(str, errors)))
+    for line in map(str, errors):
+      print line
+    return 1
+  return 0