error handling inside triggers

Hello.
I'm trying to use a "raise_application_error" inside a trigger to halt all execution and return an error. It's not working though, here is what i got:

CREATE OR REPLACE TRIGGER iva_trigger
BEFORE
INSERT OR UPDATE OF cat_iva,descrição,imposto ON Iva FOR EACH ROW

DECLARE

...
...
IF (cat_aux=:new.cat_iva) THEN
RAISE categoria_existente;
END IF;
EXCEPTION
WHEN categoria_existente THEN
RAISE_APPLICATION_ERROR(-20001,'....');

This trigger is being called by a function i defined in a package, just before an INSERT command. The problem is, instead of halting execution, it returns to the aforementioned function, and tries to execute the INSERT command.
Obviously, i don't want to execute this INSERT, since it will violate a PRIMARY KEY constraint.
Why isn't this EXCEPTION working?
How do i do it?

Thanks all!