mattdrayer/api-roles-opaque-keys: Fixed unhandled DoesNotExist exception

@chrisndodge @martynjames -- here's an initial attempt at a fix for ticket 1974 -- I've added support for converting a new-style id back to the old-style in the case of a failed lookup.  Check out the test logic -- I'm creating a course that has no corresponding forum moderator role, which helped to trigger the error condition.  I'm a little unsure of what to do if the secondary lookup for the forum role fails -- right now i"m just shrugging my shoulders and doing a 'pass'.
@chrisndodge I believe that in the version of edx-platform that we are using, the forum only uses old ids. But @aboudreault might be able to give a more accurate answer, as he knows the comments service much better than I do, and has just spent some time going over the recent PRs of the forums teams to update the XBlock -- Alan do you confirm?