Fix broken promise on next problem attempt: Only attempt to show related videos if the Exercises.RelatedVideos module is defined

Summary: ^

Test Plan: Go to a 6th-grade exercise on the iPad, and answer it correctly. Hit 'next problem', and then see the next problem!

Reviewers: mgp, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7898