Add error message on cart populate failure and don't keep re-sending the data to the server.

This was triggering many requests to be continuously re-sent in a very problematic way (server-melting), and with no user feedback.

app/assets/javascripts/darkswarm/services/cart.js.coffee

@@ -1,4 +1,4 @@
-Darkswarm.factory 'Cart', (CurrentOrder, Variants, $timeout, $http, $modal, $rootScope, $resource, localStorageService) ->
+Darkswarm.factory 'Cart', (CurrentOrder, Variants, $timeout, $http, $modal, $rootScope, $resource, localStorageService, RailsFlashLoader) ->
   # Handles syncing of current cart/order state to server
   new class Cart
     dirty: false
@@ -50,6 +50,7 @@ Darkswarm.factory 'Cart', (CurrentOrder, Variants, $timeout, $http, $modal, $roo
         @popQueue() if @update_enqueued
 
       .error (response, status)=>
+        RailsFlashLoader.loadFlash({error: t('js.cart.add_to_cart_failed')})
         @update_running = false
 
     compareAndNotifyStockLevels: (stockLevels) =>

config/locales/en.yml

@@ -2438,6 +2438,10 @@ See the %{link} to find out more about %{sitename}'s features and to start using
     resolve_errors: Please resolve the following errors
     more_items: "+ %{count} More"
     default_card_updated: Default Card Updated
+    cart:
+      add_to_cart_failed: >
+        There was a problem adding this product to the cart.
+        Perhaps it has become unavailable or the shop is closing.
     admin:
       enterprise_limit_reached: "You have reached the standard limit of enterprises per account. Write to %{contact_email} if you need to increase it."
       modals: