title: Shopware: cancel the addition of product to the cart conditionally
body: <p>I'd like to implement a feature that makes an external API call then depending on whether the request is fulfilled the product line-item gets added to the cart else it shows a translated flash message to the customer that the product couldn't be added to the cart for X reason/s.</p>
  <p>What I have tried is subscribing to the <code>BeforeLineItemAddedEvent</code> but it seems that the line item is already added to the cart by this point and I'm not quite sure how to implement the flash message thing.</p>