Cart link doesn't respect Availability Checker

I'm not sure what the correct course of action should be here, to be honest. On the one hand, availability checkers should be respected, but on the other hand, how they are respected can be context dependent.

I believe this is why, for example, the cart manager itself does not check availability before creating an order item. Additionally, I'm pretty sure Commerce only checks availability after the add to cart has occurred via the order processor, but I could be wrong.

In this case, I'd almost suggest we leave the default behavior as is but optionally execute availability checking first, rendering the message upon failure. (The problem is, that message is pretty non-specific ... but it's the best we have atm.)

Generally speaking, the availability system needs work. Will link to this from our internal Commerce channel for discussion in a future breaking release.