Add leniency for uninterpreted guesses

Summary: If we go through all formats, and none are even interpreted, mark as ungraded and show a message to that effect rather than marking as wrong. Should catch all the cases where people accidentally put in too many /// or text ("three over five").

Test Plan:
Open up a Text Input, ...
1) type in "three" and see that it shows the message and marks empty.
2) type in a wrong answer (3) and see that it marks as wrong.
3) type in the right answer and see that it marks as correct.

Reviewers: eater

Reviewed By: eater

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7437