Add check for ssl.CertificateError in netutil.py

Rather annoyingly, in the python2.7 that ships with the soon to be released Ubuntu Raring, they seem to have backported ssl.match_hostname, but they did not backport ssl.CertificateError. This causes the check here to not do what it's expected to do. This is just a trivial add to make it check for ssl.CertificateError explicitly.