About making system exceptions more user friendly there is rather
old and somehow naive article in Russian
http://www.ibase.ru/devinfo/customex.htm. Idea is rather simple, you
can skip text and look at script and usage examples. When it was
discussed I objected - such a method should provide many lock
conflicts on this pig-exception, but author (he is my friend, I know
him as truthful person) swears he use it on heavy enough loaded server
and never encountered lock conflict on exception processing. Seems in
real life, for well-designed application which operate in short
read_commited transactions, probability of coincedence of exceptions
produced by different users is low. I did'nt tried this myself, though
:)

Best regards,
Alexnder