Method: Validate all items in the cart and get a list of errors.

Method: Generates the error message for out of stock products and adds product names to it.

Method: Takes a string describing the type of stock extension, whether there is a single product or multiple products causing this exception and returns an appropriate error message.

Method: Makes the cart and sessions available to a route by loading them from core.

Method: Based on the core cart class but returns errors rather than rendering notices directly.

Method: Based on core `set_quantity` method, but validates if an item is sold individually first.

Method: Validate all items in the cart and check for errors.