No-show missing "related video" from exercise view.

This pull request is dependent on #211 and #215, and resolves #204.  Please address #211 and #215 and allow them to be merged before reviewing here.

This is a simple change that detects missing and unexpected video slugs from an exercise's 'related' list.  It then removes them from the list of videos sent to the template logic.

I also tweaked the template logic a bit, to remove the entire "Stuck? Watch a video" area when no videos are available. This is consistent across exercises that will never have related videos, and those that would if they were downloaded.

![screen shot 2013-07-09 at 7 55 48 am](https://f.cloud.github.com/assets/4072455/768784/b10ae2c4-e8a7-11e2-8bda-016a6e7f4842.png)