MySQL error code: 1193. Unknown system variable

Ok, so I'm working on triggers and it tells me this (MySQL Workbench 5.2) doesn't recognize this variable.
* Error code: 1193. Unknown system variable error_msg_3 *

I think it would be correct to use it in a trigger please help me

CREATE TRIGGER controlla_MaxCARDINALITA_INSERT
BEFORE INSERT ON SENTIERO__HA__TAPPA
FOR EACH ROW 
BEGIN

DECLARE max_cardinalita INTEGER;
DECLARE error_msg_3 CONDITION FOR SQLSTATE '99003';

SELECT COUNT(*) into max_cardinalita
FROM SENTIERO__HA__TAPPA
WHERE IDsentiero=NEW.IDsentiero;

IF max_cardinalita>=10 THEN
        SIGNAL error_msg_3;
        SET error_msg_3='INSERT: Il sentiero ha già il massimo numero di tappe consentito';

END IF;

END$$

      

        
        
        
      

    

EDIT ::

I tried this and it seems to work

DECLARE msg VARCHAR(255);
set msg = concat('MyTriggerError: Trying to insert a negative value in trigger_test: ');
signal sqlstate '45000' set message_text = msg;