Cannot place order if gift item out-of-stock

Description

Precondition:

• There is a gift promotion. For example, spend $5 and get a gift.
• The gift variant in stock is low or zero.

Steps to reproduce:

1. Go to the Quicksilver sample site.
2. Add to a cart an item satisfying a gift promotion -> The gift item is added to the cart.
3. Place the order.

Expected:
Per COM-2035, gift promotion is affected by gift's instock. Order should be placed successfully whenever gift's instock is 0 or lower than gift quantity in cart.

Actual:

• If a gift's in stock is lower than the gift quantity in cart, the system informs you with the message "The quantity of product XXX has changed". The order cannot be placed.
• If a gift's in stock is 0, the system informs you with message "The product XXX is sold out and was removed from your cart". The order cannot be placed.