writing exception handler - sql

Question

Im trying to create an exception handler to prevent null values from being inserted. I also want to make it to where if it is a null value being inserted then the original cost will display inside the message instead.
my two tables are..
create table pitem,
(item_id number,
item_cost number(10,2)
);
and..
create table pitem_audit
(item_id number,
message char(80)
);
and heres the procedure..
create or replace procedure update_item_cost
(iItemId INTEGER,
fNewcost NUMBER) AS
fCurCost NUMBER(10,2);
missing_cost EXCEPTION;
BEGIN
select item_cost INTO fCurCost from pitem
where item_id=iItemId;
IF fCurCost IS null THEN
RAISE missing_cost;
ELSE
UPDATE pitem SET item_cost=fNewcost
where item_id=iItemId;
END IF;
COMMIT;
EXCEPTION
WHEN NO_DATA_FOUND THEN
INSERT INTO pitem_audit VALUES (iItemId, 'Invalid Item identifier.');
COMMIT;
WHEN missing_cost THEN
INSERT INTO pitem_audit VALUES (iItemId, 'Item cost is null.');/*<---heres where I want it to use the original cost of the item saying 'Null cost replaced by original cost of whatever the orignal was*\
COMMIT;
WHEN OTHERS THEN
ROLLBACK;
INSERT INTO pitem_audit VALUES (iItemId, 'Miscellaneous error.');
COMMIT;
END update_item_cost;
/
/*THIS IS THE BLOCK USED TO CALL THE UPDATE_ITEM_COST PROCEDURE AND ALSO HOW I ENTER THE VALUES FOR COST*\
DECLARE
item_ident number;
cost number;
BEGIN
item_ident := 10;
cost :='';
update_item_cost(item_ident, cost);
END;
/
My question is how would I go about writing the exception handler to prevent null values from being entered..should I just do an ALTER TABLE statement and add a NOT NULL constraint? Also how would I get the message under the pitem_audit table to display 'Null cost replaced by original value of [original item_cost]'?
thanks in advance :D