Fix problem with peer grading list not rendering, test

@brianhw @nedbat Please review.  Fixes https://edx-wiki.atlassian.net/browse/OEE-139 .  Basically, the way xmodules used modulestores was changed, and the affected code was buried in a broad exception block, so it never came up as an issue here.  Fix that, and add in a test to prevent a future problem.