Corrected pylint metric status code check

This corrects the issue that was preventing the tehuti Pylint Metric from working. The problem arises because Pylint does not uniquely use `0` as the status code indicating successful execution. Instead status codes `1` & `32` indicate some form of non-recoverable error but `0` and 2<sup>n</sup> values in the range `2 <= 16` are bit-ORed so that the status code indicates what pylint errors were detected.

This is detailed at http://lists.logilab.org/pipermail/python-projects/2009-November/002068.html and in the pylint help (though note that `pylint --long-help` is required to display the status codes details).

I've also taken the opportunity to add a warning if a non-recoverable status code is encountered.