[FIX] website_sale_delivery: avoid selecting unavailable delivery method

If somebody selects a delivery method not matching any criteria, the server would crash (500 for the user). This is due to error raised in get_price_from_picking which is triggered by the function field 'price' on the 'delivery.carrier' object.
To avoid this, add field 'available' on the carrier that will be False when no delivery gird is found or when the error is raised. In this case, the delivery choice is disabled.
Also fix the computation of get_price which uses unit of stock quantity (instead of unit of measure quantity), opw 610210