The `#create_if_not_exists` function is supposed to create a repository
if it doesn't exist yet. It may happen though that the existence-cache
may pretend a repo doesn't exist yet even though it does, and in that
case we'd call the CreateRepository RPC. This worked alright until now
given that this RPC call is idempotent and simply reinitializes the repo
in case it exists already. But Gitaly is going to change semantics of
the RPC such that it raises an error if the repo exists already, where
it then won't touch the preexisting repo at all.

Prepare for this change by introducing a new `RepositoryExists` error
which gets raised when `#create_repository` returns an AlreadyExists
error. This error is rescued from in `#create_if_not_exists` such that
semantics of this RPC call remain the same.