Raise Application_Error

What is Raise Application_Error

The procedure RAISE_APPLICATION_ERROR lets user to issue user-defined ORA- error messages from stored subprograms. That way, user can report errors to your
application and avoid returning unhandled exceptions.

the syntax is
raise_application_error(error_number, message[, {TRUE | FALSE}]);

DECLARE
num_of_employees NUMBER;
BEGIN
  SELECT COUNT (*)
  INTO num_of_employees
  FROM employee;

  IF num_of_employees < 100
    THEN
/* Issue your own error code (ORA-20509) with your own error message. */
    raise_application_error (- 20509, 'Expecting at least 100 employees');
  ELSE
    NULL; -- Do the rest of the processing (for the non-error case).
  END IF;
END;