Check for forum data errors in LMS

Some messages generated by the comments service are not readily
translated because they come from third-party libraries. Thus, we plan
to try to avoid showing any comments service generated message to the
user. This check preempts the only end-user-visible CS-generated error
message that we are presently aware of.
Merge pull request #2585 from edx/gprice/lms-forum-error-check

Check for forum data errors in LMS