Consider adding a dedicated error code for GETL/UNL on locked items

This may be a duplicate (I did a search for an existing bug under a variety of different terms, but couldn't find any).

Basically there's no way for an application to disambiguate ETMPFAIL responses in cases where an item is locked, and in cases where there's actually a lot of load on the server. While in some perspective they may be the same (client should back off a bit and try later), in practice applications might treat a resource-constrained server and an already locked item a bit differently