Fauxton: Fix error if anon. user tries to create Admins

This fixes the error message if people try to create an admin without being an admin.

I also DRY'd up the code a little bit and I am using the server response here instead of duplicating it. But there are still some errorstrings needed on the clientside, e.g. for the immediate validations on the client.

I am happy to rebase/squash, if you want me to.