Prevent uncaught exceptions for ajax calls.
The two routes /checkout/cart/ajaxUpdate and /checkout/cart/ajaxDelete both require a
valid form_key. If the validation check fails an exception is thrown which is uncaught,
thus showing the customer the report page (and potentially triggering an annoying e-mail
to your developers).
Since the ajax actions already support error responses in the json object, we added an event observer to detect an invalid form_key and return a json object instead.
Pull requests welcome.