setuptools nosetest command breaks build_ext due to setup.cfg configuration not being respected
Merge pull request #790 from mr-c/fix/nosetest-setup.cfg-ignore

setuptools nosetest command breaks build_ext
Fix a traceback when using string exceptions.

We need to check that the first argument to `issubclass()` is a class
before using it, since we have the possibility that it's a string.