Can't catch exception in form from server

Question

The Block is built on procedure.
Use code in procedure of the blocking:
SELECT id
INTO temp
FROM tab
WHERE id = n_var --variable
FOR UPDATE NOWAIT;
EXCEPTION
WHEN OTHERS THEN
RAISE_APPLICATION_ERROR(-20001, 'ERROR');
Open two copies of form.
In one form change record. In the other form in an effort change record appears the error with code -20001.
This is correct.
If the form does not contain the trigger ON-ERROR - a code of the error -20001.
If add the trigger ON-ERROR:
IF DBMS_ERROR_CODE = -20001 THEN
MESSAGE('RECORD IS LOCKED');
MESSAGE('RECORD IS LOCKED');
END IF;
I get the error with code -1403.
Why? When i delete trigger - error code -20001.
How do I get the code of the error -20001 in trigger ON-ERROR. Why DBMS_ERROR_CODE = -1403 if in procedure i generate error with code = -20001???

Answer 1

I don't have another sql query.
All work correct while i don't add trigger ON-ERROR.
Without this trigger, form work is correct. I get error from server with code -20001. When i add trigger ON-ERROR - in this trigger dbms_error_code = -1403. Why? How error code may depend from this trigger?