Fixed #14503 -- Unified multiple implementations of test cases assert* methods...

Fixed #14503 -- Unified multiple implementations of test cases assert* methods that verify a given exception is raised by a callable throughout the Django test suite.

Replaced them with a new assertRaisesMessage method of a new SimpleTestCase, a lightweight subclass of unittest.TestCase. Both are also available for usage in user tests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16610 bcc190cf-cafb-0310-a4f2-bffc1f526a37