Send a new event from interface.js whenever a helpful message is shown after user attempts a problem (e.g. 'It looks like you haven't answered the entire problem yet').

Summary: Event will be used by mobile.

Test Plan:
Drop into mobile repo w/ my prototype that listens for this event
Verify that event is fired and sends appropriate message along.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D10466