* Registers the Github AcocuntSuspended & RepositoryUnavailable errors
Adds the above-mentioned errors to the list of known errors raised by
Octokit as to make possible for the rest of the application to
specifically trap those.
* Makes the RepoSyncWorker error handling rely on types
The current implementation attempted to parse the error message to
determine if the error reported by Github's API can/should be handled,
this commit changes the implementation so that the exception type is
used instead.
