URGENT !!! Database Trigger Issue

Hi,
I am trying to create a DB trigger on PO_DISTRIBUTIONS_ALL ( Oracle Applications) table to pop up a error/warning messages when the above table column attribute1 is updated through the form DFF. But I am getting nothing

the trigger code is as below:

---------------------------------------------------------

CREATE OR REPLACE TRIGGER APPS.XXX_GENERATE_CHARGE_ACC_T1
BEFORE UPDATE OF
ATTRIBUTE1
ON PO_DISTRIBUTIONS_ALL
REFERENCING
NEW AS NEW
OLD AS OLD
FOR EACH ROW
WHEN (
(NEW.ORG_ID = 102 AND OLD.ATTRIBUTE1 != NEW.ATTRIBUTE1)
)
declare
v_project varchar2(7);
v_ccid number := NULL;
v_old_ccid number := NULL;
v_gl_acct_number varchar2(6) := NULL;
v_old_segment1 GL_CODE_COMBINATIONS.segment1%TYPE := NULL;
v_old_segment2 GL_CODE_COMBINATIONS.segment1%TYPE := NULL;
v_old_segment3 GL_CODE_COMBINATIONS.segment1%TYPE := NULL;
v_error exception;
Begin
v_project := :NEW.ATTRIBUTE1;
v_old_ccid := :OLD.CODE_COMBINATION_ID;

IF v_project != '0000000' THEN
BEGIN
<< Some validations >>
END;

:NEW.CODE_COMBINATION_ID := v_ccid;
:NEW.VARIANCE_ACCOUNT_ID := v_ccid;

ELSIF v_project = '0000000' THEN
raise_application_error(-20003,'Project Number is required', TRUE);

END IF;

End;
---------------------------------------------------------------
The exepectation is that whenever user updates attribute1 as 0000000 from the form using the DFF and tries to save, the error/warning message should appear in the form giving an error note.

But in this case, nothing happens upon clicking the save button. the form just stays the same and apparently the data is not saved also.

This is a standard form, and we donot want to customize the form.
Is there a way to display error/note/warning message when the updated data is 0000000.

Any help would be greatly appreciated.

Thanks in advance.