Show why an add to cart was rejected instead of doing nothing

[boo-code]

Questions	Answers
Description?	Adding a product to the cart from the product page when it has just gone out of stock does nothing visible: no modal, no message. The server answers correctly - CartController returns {"hasError":true,"errors":["This product ... is no longer available."]} - and themes/_core/js/cart.js turns that into prestashop.emit('handleError', {eventType: 'addProductToCart', resp}). Nothing in this theme listened, so the message was dropped and the reason only surfaced after a reload. Hummingbird already registers such a listener; classic never did. This adds it.
Type?	bug fix
BC breaks?	no
Deprecations?	no
Fixed ticket?	PrestaShop/PrestaShop#16260
Sponsor company	-
How to test?	Set a product to out of stock with "Deny orders" and open its product page in this theme, then click "Add to cart". Before this change nothing happens and prestashop._events.handleError has 0 listeners; after it, the error text from the server is displayed. A product that is in stock still adds normally.

Measured on a 9.2.x shop with classic 3.1.2, on upstream/develop's own build:

prestashop._events.handleError                    -> 0 listeners
click "Add to cart" on a now-out-of-stock product
  events emitted                                  -> ["handleError"]
  #product-availability                           -> unchanged
  .alert-danger                                   -> none
  page text contains "no longer available"        -> false

Only _dev/js/product.js is changed; no built asset is in the diff.