Diff-quality should not fail on pylint warnings.

@Bachmann1234 

After working around #115, we later encountered a deprecation warning because we had an old setting in pylintrc. (See https://github.com/edx/edx-analytics-data-api/pull/32).  This is because pylint pipes warnings through stderr (which is no shock of course). This was causing diff-quality to fail. 

This change will only fail diff-quality if pylint pipes errors AND ends with an exit code that's not 0. I've added and updated tests to reflect this.
Can you update this test to actually have something in stderror? It exercises the code a little better. Because right now this test will hit the conditional but short circuit before you get to the check you added.

make sure it is a byte string