Add check for ssl.CertificateError in netutil.py

Rather annoyingly, in the python2.7.x that ships with the soon to be released Ubuntu Raring, they seem to have backported ssl.match_hostname from python3.2+, but they did not backport ssl.CertificateError. This causes a check in netutil.py to not do what it's expected to do. This is just a trivial add to make it work.