raises matcher Mismatch text could be more helpful

[thomir]

I often write test code like so:

self.assertThat(
    lambda: client.add_key_for_owner(user_two, fingerprint),
    raises(GPGServiceException("Fingerprint already in the database.")))

However, if there's a bug in my code and the function in question raises a different exception, I get the following output:

MismatchError: <type 'exceptions.NameError'> is not a <class 'lp.services.gpg.interfaces.GPGServiceException'>

At this point I usually change the test to run the function outside of an assertion so I get a proper traceback that allows me to debug the problem. It would be nice if the raises matcher gave me more verbose information about what was raised - at a minimum str(exception).

[jml]

Yeah, I agree that this would be a better behaviour. ISTR @rbtcollins suggesting that the current behaviour is a feature, not a bug. Rob, want to chime in?

[mithrandi]

I can't think of any situation where I would want to only see the type of the incorrect exception.