Rename Yt:Errors::MissingAuth to Unauthorized

This is in order to match as much as possible the Net::HTTP error
names, since we will be handling new errors (e.g. Forbidden) soon.
Raise Yt::Errors::Forbidden if YouTube returns 403

For instance, if you try to get the earning of a channel for which
you are not the CMS content owner.
Merge branch 'unauthorized-and-forbidden-errors'