Raise exception in Trigger after update

 This does not seem to work. I want to raise an exception after the update or insert on table. Update does not occur.

create or replace sometrigger

after update or insert on table_name

for each row

declare

begin

   check_table_condition(:new.column_name);

--if proc throws exception

exception

  when others then

   RAISE_APPLICATION_ERROR(-20500, 'some message', TRUE);

end sometrigger;