create or replace PROCEDURE testException( a in varchar2) as
    INPUT_ARGUMENT_NULL EXCEPTION;
BEGIN
    IF a is null then
        raise INPUT_ARGUMENT_NULL;
    END IF;
    dbms_output.put_line('No Exception raised');
EXCEPTION
    WHEN INPUT_ARGUMENT_NULL THEN
       raise_application_error(-20001,'INPUT ARGUMENT PASSED AS NULL');
END testException;

SQL> EXECUTE TESTEXCEPTION(NULL);

begin TESTEXCEPTION(NULL); end;

ORA-20001: INPUT ARGUMENT PASSED AS NULL
ORA-06512: at "PTTDEV01.TESTEXCEPTION", line 10
ORA-06512: at line 1