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.