handle failures in ajax post requests
handle failures in ajax post requests

currently, when the server's timing out, the page will hide buttons and just sit in an "in progress" state, needing a refresh to work again.

openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server,openfoodfacts/openfoodfacts-server