Issue warning when certificate has no subjectAltName

Related bug: #497
Tweaked phrasing/formatting.
More phrasing tweaking.
Update tests with new certificate for full coverage

Also add instructions for future selves needing to generate new certificates and pem files
Merge pull request #499 from sigmavirus24/deprecation-warning-when-no-subjectAltName

Issue warning when certificate has no subjectAltName.