SRC 118666: [eCost] Shopping Cart: An error occured validation message displayed if non-existing sku's are quick added

Problem: adding skus that does not make sense or is not in the inventory, like "1", "66",  or "454908" results in NPE and an incorrect error message ("problem occurred while processing your request")

Solution: Fix the NPE and provide additional checking for items like checking if the item is in the inventory and if in stock in eCost, and then return appropriate eror message, like displaying the message "the item is out of stock" for items that are not in the catalog, and "SKU is not in the inventory" for items that are totally not in the database.