Fix broken repo errors in the UI

This should prevent repo errors (or 404s) in the UI, together with https://gitlab.com/gitlab-org/gitlab_git/merge_requests/124

The `exists?` cache is now expired if the repo gets broken. 

Related MR: https://gitlab.com/gitlab-org/gitlab_git/merge_requests/124

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20501

See merge request !6491