Fix error adding user in backstage

Adding a user and failing the validation rules will give a page with the wrong markup.

backstage/users.php
- Added header for adding a user validation.

include/functions.php
- Add a line in message function to include backstage/footer.php instead of /footer.php

PS. Only like to send a pull request using the d6f125f commit but I somehow can't. I will get better at this. Sorry.