You said it was pure SQL you were using for the trigger?  Then I would check the SQL State.

Yeah, that's not the problem, I can get the error from code or state, but I can't do anything with it. I finally found that rollback and commit aren't allowed, but you can create a 'savepoint' and 'rollback to savepoint'. Problem now is, as soon as I get the error, like 'duplicate key', the process drops straight to the final 'end' statement at the end of the program. Skips over everything else, including any if statement where I could do something - rollback to savepoint - and I can't figure out any way around it.

There used to be a WHENEVER clause that was used for SQL exception/error handling. Perhaps it is still supported in SQL-based triggers.