Rescue 422 response when unsubscribing from a repo with Curry

GitHub will respond with 422 when you attempt to unsubscribe from a repo
that doesn't exist, which is raised as Octokit::UnprocessableEntity.
This rescues Octokit::UnprocessableEntity so you can remove watched
repos that have been deleted and logs the exception.
Rescue from missing Pull Requests

In the event that a repo or pull request gets deleted we shouldn't raise
an exception when Octokit 404s we should just log it and move on.
Merge pull request #427 from opscode/curry-deleted-repos

Rescue 422 response when unsubscribing from a repo with Curry