Fixed #25948 -- Added guidelines for SimpleTestCase.assertRaisesMessage() usage.

          """

          ...

* In tests, use :meth:`~django.test.SimpleTestCase.assertRaisesMessage` instead

  of :meth:`~unittest.TestCase.assertRaises` so you can check the exception

  message. Use :meth:`~unittest.TestCase.assertRaisesRegex`

  (``six.assertRaisesRegex()`` as long as we support Python 2) only if you need

  to use regular expression matching.

Imports

-------

    Asserts that execution of ``callable`` raises ``expected_exception`` and

    that ``expected_message`` is found in the exception's message. Any other

    outcome is reported as a failure. Similar to unittest's

    :meth:`~unittest.TestCase.assertRaisesRegex` with the difference that

    ``expected_message`` isn't a regular expression.

    outcome is reported as a failure. It's a simpler version of

    :meth:`unittest.TestCase.assertRaisesRegex` with the difference that

    ``expected_message`` isn't treated as a regular expression.

    If only the ``expected_exception`` and ``expected_message`` parameters are

    given, returns a context manager so that the code being tested can be