Handle deleted products in the Cart

[glennjacobs]

Following on from #721

If a product variant is deleted from the system, the cart needs to handle this. Although we could simply delete the cart line, this isn't a great UX. Maybe we need to consider informing the user somehow that a product is no longer available?

We also need to double-check that a cart cannot progress to checkout if a product is no longer available (ie. deleted).