error_on_undefined_vars doesn't work for role parameters

I was going to submit this as a bug but I decided to send email first in case I’m doing something wrong.

The session below should show all required information, but let me know whether something else is needed.

`
ansible$ cat playbooks/test_role_with_undefined_list.yml

Actually… possibly_undefined_list should have thrown an undefined error before you ever try to access list_of_items on the second run.