Fix some low hanging inefficiencies in the docs server

chrome/common/extensions/docs/server2/object_store_creator_test.py

 #!/usr/bin/env python
 # Copyright 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 unittest
 
 from test_object_store import TestObjectStore
 from object_store_creator import ObjectStoreCreator
 
@@ skipping 14 matching lines @@
 
   def testWithCategory(self):
     store = self.creator.Create(category='cat')
     self.assertEqual('_FooClass/cat', store.namespace)
 
   def testWithVersionAndCategory(self):
     store = self.creator.Create(version=43, category='mat')
     self.assertEqual('_FooClass/mat/43', store.namespace)
 
   def testIllegalIinput(self):
-    self.assertRaises(self.creator.Create, category='5')
-    self.assertRaises(self.creator.Create, category='forty2')
-    self.assertRaises(self.creator.Create, version='twenty')
-    self.assertRaises(self.creator.Create, version='7a')
+    self.assertRaises(AssertionError, self.creator.Create, category='5')
+    self.assertRaises(AssertionError, self.creator.Create, category='forty2')
+    self.assertRaises(AssertionError, self.creator.Create, version='twenty')
+    self.assertRaises(AssertionError, self.creator.Create, version='7a')
 
 if __name__ == '__main__':
   unittest.main()