Custom error message in validation

Hi i have this simply validation rules :

Code:

    public function mancanti_bolla($value, ?string &$error = null , array $data): bool
    {
        //dd($value);




        $error = "Error for mancanti_bolla";

        return false;


        //controllo i mancanti

    }

But in my view i have : 
Validation.mancanti_bolla instead of  Error for mancanti_bolla.

How can i set custom error message?