Out of stock messages not generated for individual order items on checkout form

Problem/Motivation

When the enforcement module is enabled, a "One or more Items are out of stock" message is generated when appropriate. The check happens in commerce_stock_enforcement_is_order_in_stock(), which accepts a $show_warnings parameter to decide whether to generate a message for each affected order item. The method is called with TRUE for the cart page but with FALSE for the checkout form. Also, the message is not repeated when there are multiple affected order items that have the same name and quantity.

Proposed resolution

Call the method without the $show_warnings parameter (which defaults to TRUE) for both cart page and checkout form, and permit the repetition of individual messages for order items.