In go-gitea#17742 it was noted that there a completely invalid git repository underlying a
repo on gitea.com. This happened due to a problem during a migration however, it
is not beyond the realms of possibility that a corruption could occur to another
user.

This PR adds a check to RepoAssignment that will detect if a repository loading has
failed due to an absent git repository. It will then show a page suggesting the user
contacts the administrator or deletes the repository.

Fix go-gitea#17742

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>