Cause catalog/product updates to update existing carts

[jedateach]

If a product is no longer purchasable, existing shopping carts should be updated, and customers notified that the product is no longer available.

This is particularly important to sort out if a store is to have long-lasting carts - eg: months, or infinite...a good idea to encourage sales over time.

Relates to issue #8 - creating a site-wide notification system.