error description for raises with wrong type is too brief

[Robert Collins]

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  File "testrepository/tests/test_testcommand.py", line 123, in test_TestCommand_cleanUp_disposes_instances_fail_raises
    raises(ValueError('Disposing of instances failed, return 1')))
testtools.matchers._impl.MismatchError: <class 'TypeError'> is not a <class 'ValueError'>
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It would be nice if it listed the actual backtrace here, so I could see where teh TypeError was in fact raised.