If the first entry found in the oplog is greater than the token provided by the command, return an error that indicates that most likely the op that was requested has rolled off the back of the oplog.
If the first entry found in the oplog is less than the token provided by the command, but no entry with the exact token was found, then that means something is more seriously wrong - since we only return committed entries that can't roll back, such a scenario should be impossible