drop procedure test_raise_exception;

create procedure test_raise_exception(vMsg varchar( 255 ))
returning
  varchar( 255 ) as Msg; 

define vSQL, vISAM integer;
define vIfxMsg     varchar( 255 );

return vMsg with resume;

BEGIN
  on Exception  SET vSQL, vISAM, vIfxMsg
    return vIfxMsg;
  end exception;

  raise exception - 746 ,  0 , vMsg;
END;
end procedure;

msg
 123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890 
 123456789012345678901234567890123456789012345678901234567890123456789012 

execute procedure test_raise_exception('123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890')