IBO 4, BCB5, FB 1

I try to catch errors, that occure when user tries to post record
with missing mandatory fields, so I could try to fix them on the fly
or give user some menaingful messages. I tried to use TIB_Query
OnError event, but it doesn't get called - I see IBO
errormessage: "FIELD1 is a rfequired field", but I can't catch it .

What event should I use to handle these errors?

Riho Ellerma