ORA-688

[ORA-688](https://openedx.atlassian.net/browse/ORA-688): Student receiving "Unable to Load" error message

Given the Splunk logs at the time of the error, I believe that the "unable to load" message is due to a KeyError in the grade section.  The only way for this to occur is when the rubric is modified after an assessment is created.  We prevent course authors from doing this post-release, but it's possible for authors to circumvent this using course import.

This is not ideal, since the students won't see a score for criteria that are no longer part of the problem definition, but I don't know another way to handle this reasonably.  We need to ensure that all the assessments have the same criteria (or else the layout could break), and we need to allow course authors to fix typos post-release.

@stephensanchez thoughts?