Grouped platform-specific system error message regex-es.

The two files contain the couple of platform-specific error message-matching regex-es I have found so far with more to be added as we go along.

These provide an easier and more standardised means of checking for those specific error outputs generated by failed system calls using gc.Matches and gc.ErrorMatches.

This pull depends on the UserNotFound error type included in [this](https://github.com/juju/errors/pull/14) pull request.

(Review request: http://reviews.vapour.ws/r/547/)