Appearance settings can be left in broken state

If you go to /admin/appearance and set only navigation bar logo, settings will be left in broken state - navigation logo will be saved in DB but not in the filesystem, because of validation errors.

Also, is is a UX problem, as there is no indication that sign in title/description are required.

Screen_Shot_2017-03-10_at_20.19.11

GitLab 8.16.6, all health check info posted before in https://gitlab.com/gitlab-org/gitlab-ce/issues/29004