Course Rerun: include stack trace in errors and Log errors.

@cpennington @dmitchell Please review.

This PR changes the `CourseRerunUIStateManager` django model to persist the exception stack trace in addition to the message, in the event of Failures.
Is this only called inside an `except` clause? I don't think you need to include exception.message yourself (I think that `format_exc` does that), but I'm worried that it won't actually format an exception if you aren't inside an exception handler.