Catching errors in pl/pgsql

Hi everybody!

I'm migrating severals application from Oracle to PostgreSQL. I read all
the documents about PL/PGSQL and how porting from oracle to pg. I ported
several procedures succesful, but I have a lot of problems "translating"
this kind of code:
(...)
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
V_Error := 'PR_INSERT_VIOLATION_PK';
WHEN E_Parent_Not_Found THEN
V_Error := 'PR_INSERT_PARENT_NOT_FOUND';
WHEN OTHERS THEN
V_Error := 'PR_INSERT_OTHERS';
(...)

IF vError IS NULL THEN
COMMIT;
ELSE
ROLLBACK;
RAISE_APPLICATION_ERROR(-20000,V_Error);
ENDIF;
(...)

How I can catch the error, for example, if I try to insert a duplicate
value on primary key? or if parent keys not found? I need to catch the
error inside the pl/pgsql function and then pass a message to
application. I will rise a exception (I know how to do this) but I don't
know how to identify the errors...

Thanks in advance!

--
Fernando O. Papa
DBA