Be more resilient in the case of missing omniauth settings

Created by: rspeicher

Should no longer freak out when omniauth settings aren't present in gitlab.yml. People who aren't using it shouldn't even have to put a 'false' entry in their config for it (and probably wouldn't, after an upgrade).

Should prevent a few future support requests where people who have no interest in using Omniauth and don't bother to update their config settings and suddenly get errors.

Also I hate method definitions that have arguments but no parenthesis, and rescuing from all exceptions when you really only needed one, so I changed those.