[1.8.x] Clarified assertRaisesMessage() behavior with respect to the message.

            SimpleTestCase.assertRaisesMessage(expected_exception, expected_message)

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

    that the exception has an ``expected_message`` representation. Any other

    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.