Show action form errors using message_user

Not very neat, but way better than nothing when there are errors
(previously the error was invisible and the action simply seemed to have
completed).
Change submit name to make edit_permission work

Turns out that since Django 1.2 the admin only invokes actions when
handling POST if the POST data specifically *doesn't* contain `_save`:

https://github.com/django/django/commit/cce32a9b0900210d524b80160209c3e2b5dfb156

So this submit name was preventing `edit_permission` from acting on the
POST data since Django 1.2.
Merge pull request #29 from gthb/fix-edit_permission-action

Fix edit permission action