How I can send a Message Error From My Sql Procedure?

See more:

C#

MySQL

Hello Everyone,

I am making a transaction procedure in my sql, but I want to send a message-error
If the register already exists before I insert.

For example:

CREATE DEFINER=`root`@`localhost` PROCEDURE `AsigPolizClau` //.......--->more code
  BEGIN
  
  START TRANSACTION;
  
    IF((SELECT id FROM poliza where field1=fk_field1) IS NULL) THEN    
      BEGIN
        /*
          ...
          ...
        */
      END;
   ELSE
      BEGIN
       //HERE I WANT TO SEND A MESSAGE ERROR!!!!!!!! (HOW????????)
       //HERE I WANT TO SEND A MESSAGE ERROR!!!!!!!! (HOW????????)
       //HERE I WANT TO SEND A MESSAGE ERROR!!!!!!!! (HOW????????)
      END;
   END IF;

   SET a=(SELECT @@error_count);
   IF a > 0 THEN
    ROLLBACK;
   ELSE
    COMMIT;
   END IF;   


  END

I want that the procedure finished and send a message error to my
MySqlCommand variable in C#.
and I give a message from my C# code, maybe a message pased from sql procedure to
the procedure in c#.

It is posible???

Thanks in advance!!