Student/teacher custom form errors should be updated

[willgearty]

From @kkbrum:

Now that we have more user friendly ways of setting this tag, perhaps we should have a more helpful message here if the current user is an admin (ie, give them a few instructions and lead them to the custom forms page). In particular, they really don’t need the “learn_extraform_id” wording anymore since they shouldn’t be going to the admin pages and setting that