Paver: fail if pylint/pep8 violations threshold is breached.

Hmm I'm looking at the all-tests.sh scripts again. I ddin't realize we call these tasks separately, so I suppose the point is half-moot.

The still-relevant part is that if you have a pep8 violation (for example), the build will fail and you won't see if you have any pylint or diff-cover/quality issues until you run a new build. Perhaps that's best solved outside of the context of paver?