Don't error out if a datagrid is sorted on a missing column.

At some point we had mistakenly marked the "people" column as sortable, which
doesn't really make sense. We've since fixed this, but if users had "people"
left in their sort preferences, it would cause an uncaught exception. This adds
a none-test and logs the failure.

Testing done: Could load /r/ using my old db.

Reviewed at https://reviews.reviewboard.org/r/5688/