handling exceptions, really not simple... :-((

Hi 2 everybody!

I'm evaluating a database that supports our 3-tier solutions, in addition to
Oracle.
Postgres is at the moment the candidate n. 1, but I realized it's very hard
to translate this type of code (Orqcle PL/SQL):

FUNCTION F_EXT(par1, par2 ...) RETURN.... IS
BEGIN
SAVEPOINT spF_EXT;
...[instructions]
INSERT INTO...
UPDATE...
DELETE...
...[instructions]

SAVEPOINT spF_INT;
ret := F_INT(par1, par2 ...);

IF ret = 'KO' THEN
ROLLBACK TO spF_EXT;
ELSE IF ret IS NULL THEN
ROLLBACK TO spF_INT;
END IF;

...[instructions]
COMMIT;
EXCEPTION
WHEN.....
...[instructions]
END F_EXT;

The goal to achieve is to trap every kind of execution errors without trying
to prevent their occurrence.
In any case, I usually can't be sure the function (F_EXT) works correcty: an
exception could be lanched by an internal function (F_INT), I couldn't know
to estabilish what happens and where ...
By the way, the evironment is:
the browser requests a PHP page that connect to a Data Source (Oracle) and
lanch a Stored Procedure; after the execution by Oracle, PHP get the result
and returns data or a message to the client.
It's very important for me to solve the application logic in the Back-End of
the system (Oracle or Postgres): integrity and meaning of function aren't
only "commit all" or "rollback all", and when something of wrong happens, I
need to know the type of exception and when it occurred. In Oracle I can do
this, but in Postgres I've not yet found anything of similar.
Can anyone help me?
Thanks in advance...

Eaglet