Invalid sort-specifications are silently ignored

As noted in UIU-101, it seems that when submitting a query such as

active="true" sortby noSuchField personal.lastName personal.firstName

to mod-users, the resulting list of users is sorted by name (in lastname, firstname order) with the invalid noSuchField key being completely ignored.

Instead, the query translator should adhere to the FEFO principle (Fail Early, Fail Often), and reject the query with an explicit error message along the lines of "unrecognised sort-key 'noSuchField'".