Right now, this code:

    class FooError(Exception): pass
    class BarError(Exception): pass

    def test_me(self):
      with self.assertRaises(FooError):
        raise BarError("something")

will have the error "BarError: something" with no indication that an exception was expected but just that we got the wrong one.

It would be help to change the message to something like:

  Expected exception of type FooError but exception BarError('something') was raised.