Simple error "exception when" in trigger

It looks like exceptions are not raise in triggers as expected.

example:

1. trigger function

BEGIN
  EXCEPTION
         WHEN OTHERS THEN
               RAISE NOTICE '% already exist.',NEW."UNAME";
END;

I've tried a before and after trigger
I've also tried the sql error codes for a violation of a UNIQUE
constraint but still no luck.

The the examples of the manual every example have a INSERT or UPDATE
statement before the
EXCEPTION WHEN and it's not done within a trigger.

I would love to make it "GLOBAL" so it does not matter if the INSERT
or UPDATE get called by a FUNCTION or not, thats why I want the
constrain violation to be trapped within a TRIGGER.

Is this possible or not ?

Thanks a mil.