Rescue 422 response when unsubscribing from a repo with Curry

:construction: @bcobb this resolves the surface level problem on [this card](https://trello.com/c/hOl4u5Fd/187-curry-fails-when-you-unwatch-a-repository-that-has-already-been-deleted) but I noticed there are some other exceptions related to watched repos that don't exist anymore mostly 404 errors from Octokit. I'm wondering if we also want to address those issues or figure out a way to have GitHub remove watched repos from Supermarket when they're deleted from GitHub presumably that would also solve all the problems, though I'm not sure if this is possible.