Improve handling of Github errors

[marcusmoore]

This is rare but Github doesn't always return JSON when we're expecting it and throws an uncaught exception.

One place this happens is when the SyncPackageRepositoryData job is called. We end up calling app/GitHubRepo::latestRelease() which returns HTML that includes the string We had issues producing the response to your request. Sorry about that. Please try refreshing and contact us if the problem persists.

We can probably check to see if we got an error response and send another request to see if it was just bad luck.