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

This is ported from the Jenkins Violations plugin (https://github.com/jenkinsci/violations-plugin), specifically, https://github.com/jenkinsci/violations-plugin/blob/d5b4d6d7cd4cafa5e253be6abe3f2f3d27ab90cd/src/main/java/hudson/plugins/violations/types/pylint/PyLintParser.java#L35

I have tested it on our violations and out of more than 4000 violations, we come up with an exact match on the number of violations. 

I could also just say this is ported from the Jenkins plugin if that is easier.