Implement RepositoryExistsException

This exception is thrown when we try to create a repository but it
already exists on the server.

Wanted to get some eyes on this early. This is ready to merge. I plan to take the same approach for:

1. `OverPrivateRepositoryQuotaException`
2. `CannotCreatePrivateRepositoryOnFreePlanException` (not sure this is doable in Octokit.net yet).

^^^ I could use help naming those better. :)