A recent bug  reported as part of the translation effort alerted me to the fact that we
have a lot (and I mean a LOT - over 100 per file) of deprecated, unused keys in the
various AppErrors files that serve no purpose and just take up space and waste translators
time when they examine them.
To make a long story short - I've just merged a patch to remove all these useless
messages, and enforce via unit tests that EVERY key there should have a corresponding
constant in the EngineMessage or EngineError enums.
Many thanks to my reviewers!
I know this was an tedious patch that couldn't have been too fun to review.