More useful error message when application stage gets desynced from group status

[adehnert]

When a group is in application stage "submitted" but group status "NGE" (or, really, anything but "Applying"), they show up on the list of group applications to review​ but error when you view the approval page​. Especially while ASA-#267 is unfixed, we should include links to the group's page and the group startup admin, to make it easier to make the state consistent again. (Making cron yell at you would also make sense.)