Refuse to handle forms without ->auth_id (but do it in the right place, and noisily)

Refuse to handle forms without ->auth_id (but do it in the right place, and noisily)

In 0b47ea1882346fdedfcd8a315f51aeb39e13459e ("Refuse to handle forms without
->auth_id"), the process_auth_form_cb for the OpenConnect CLI was modified
to silently reject forms with auth_id unset.

Issues with this:

1. If a form doesn't have its auth_id set, it'll fail *silently*, which
   makes it confusingly difficult to identify the root cause. (See #351.)
2. As that commit message says, GUIs/front-ends need the auth_id to be set,
   but it didn't do anything to enforce it other than for the CLI.

The solution is to reject forms with auth_id unset in process_auth_form()
itself, rather than expecting the front-ends’ callback functions to check
this, and to do so with an error message explaining that this is a bug in
OpenConnect.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>