Repository loading: returning proper error if not found

We already had good enough error responses for the case when the storage is not found, but it was missing for the case when the repository is not found in an existing storage.

This is important, because it can actually break the landing page of some users: the Rails application does not seem to expect an unspecified Gitaly error here, but it should recover from NOT_FOUND.

I've been impaired by this on dev setup after with some repository removals, but don't have anything of that kind on hand any more, which would be more reassuring.