SQL & PL/SQL

We are developing a new module for our Insurance system.  It has Oracle forms 11g as the front end, 11g R2 as the back-end DB.

I am trying to make the error handling as simple as possible using the KISS principle.

I created 3 SPs to simulate this:

create or replace 
procedure sub_proc is
   i number := 1;
BEGIN
   DBMS_OUTPUT.PUT_LINE('SUB_PROC. About to raise 1/0 exception');   
   i := i / 0;   
EXCEPTION
   WHEN OTHERS THEN
      DBMS_OUTPUT.PUT_LINE('----Exception Handler in SUB_PROC----');
      RAISE_APPLICATION_ERROR(-20000, SQLERRM);
END;
create or replace 
procedure top_level_proc is
  -- This proc is called by the Oracle Form
BEGIN
   DBMS_OUTPUT.PUT_LINE('top_level_proc');
   SUB_PROC();
EXCEPTION
   WHEN OTHERS THEN
      DBMS_OUTPUT.PUT_LINE('----Exception Handler in TOP_LEVEL_PROC----');
      RAISE_APPLICATION_ERROR(-20000, SQLERRM);   
END;
create or replace 
procedure oracle_form is
 -- This mimiks the Oracle Form front end
BEGIN
   DBMS_OUTPUT.PUT_LINE('oracle_form');
   top_level_proc();
EXCEPTION
   WHEN OTHERS THEN
      DBMS_OUTPUT.PUT_LINE('ORACLE_FORM EXCEPTION HANDLER: ' || SQLCODE || ' / ' || SQLERRM);      
END;

Now I run the Form (simulation):

BEGIN
   oracle_form();
END;

This is the output I get:

oracle_form
top_level_proc
SUB_PROC. About to raise 1/0 exception
----Exception Handler in SUB_PROC----
----Exception Handler in TOP_LEVEL_PROC----
ORACLE_FORM EXCEPTION HANDLER: -20000 / ORA-20000: ORA-20000: ORA-01476: divisor is equal to zero

As you can see, the Oracle Form's exception block, the exception I get is not clean. 

In the Oracle Form's exception handler, I want to pop-up a  small form

to give the user a nice small screen saying an error occurred and

also giving the error message, in this case ORA-01476: divisor is equal to zero.

Now, SQLERRM has this string: ORA-20000: ORA-20000: ORA-01476: divisor is equal to zero

How can we get only the ORA-01476: divisor is equal to zero as error?

Because, now I will have to process this string, staring with the end, and move forward

until I encounter the ORA-01476 and then get the sub-string. I might be even

able to do this using a regular expression, which I am not familiar with.

So, how to do this error handling elegantly?