For transitional constraints, you may want to refer to the old values:
, OLD( p.discount ) old_discount
FROM person p
WHERE discount > OLD( discount ) + 5
For this constraint, you can specify the message:
The personal discount for <p.first_name.value> <p.last_name.value> currently is <old_discount.value> %.
This percentage must not increase by more than 5% at a time.