Add the last of scheduled fixes for the not showing bugs

Summary: Occasionally, for no apparent reason, either the type is passed in incorrectly (especially during Mastery Challenges) or the exerciseId is just null (not even undefined; null). This makes a last ditch effort to fix those errors while still reporting them

Test Plan: On the proceeding line, set `exerciseId = null;` or `typeNum = "notaRealType";` and see that it fixes the problem without causing any new ones when that code is absent

Reviewers: alpert, eater

Reviewed By: eater

CC: eater, alpert

Differential Revision: http://phabricator.khanacademy.org/D6294