Added docs for assertRaisesMessage as context manager.

details that may help you to identify the location and cause of an failure in

your test suite.

.. method:: SimpleTestCase.assertRaisesMessage(expected_exception, expected_message, callable_obj=None, *args, **kwargs)

.. method:: SimpleTestCase.assertRaisesMessage(expected_exception, expected_message, callable, *args, **kwargs)

            SimpleTestCase.assertRaisesMessage(expected_exception, expected_message)

    Asserts that execution of callable ``callable_obj`` raised the

    ``expected_exception`` exception and that such exception has an

    ``expected_message`` representation. 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.

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

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

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

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

    written inline rather than as a function::

        with self.assertRaisesMessage(ValueError, 'invalid literal for int()'):

            int('a')

.. method:: SimpleTestCase.assertFieldOutput(fieldclass, valid, invalid, field_args=None, field_kwargs=None, empty_value='')