show specific error message when add-to-cart fails

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

@@ -50,7 +50,7 @@ Darkswarm.factory 'Cart', (CurrentOrder, Variants, $timeout, $http, $modal, $roo
         @popQueue() if @update_enqueued
 
       .error (response, status)=>
-        Messages.flash({error: t('js.cart.add_to_cart_failed')})
+        Messages.flash({error: response.error})
         @update_running = false
 
     compareAndNotifyStockLevels: (stockLevels) =>

app/controllers/cart_controller.rb

@@ -11,7 +11,8 @@ class CartController < BaseController
     Spree::Adjustment.without_callbacks do
       cart_service = CartService.new(order)
 
-      if cart_service.populate(params.slice(:products, :variants, :quantity), true)
+      errors = cart_service.populate(params.slice(:products, :variants, :quantity), true)[:errors]
+      if errors.blank?
         order.update_distribution_charge!
         order.cap_quantity_at_stock!
         order.update!
@@ -22,7 +23,7 @@ class CartController < BaseController
                        stock_levels: VariantsStockLevels.new.call(order, variant_ids) },
                status: :ok
       else
-        render json: { error: true }, status: :precondition_failed
+        render json: { error: errors.full_messages.join(",") }, status: :precondition_failed
       end
     end
     populate_variant_attributes

app/services/cart_service.rb

@@ -19,7 +19,7 @@ class CartService
       attempt_cart_add_variants variants_data
       overwrite_variants variants_data if overwrite
     end
-    valid?
+    { errors: errors }
   end
 
   private
@@ -109,10 +109,6 @@ class CartService
     end
   end
 
-  def valid?
-    errors.empty?
-  end
-
   def cart_remove(variant_id)
     variant = Spree::Variant.find(variant_id)
     @order.remove_variant(variant)
@@ -145,7 +141,7 @@ class CartService
 
   def check_order_cycle_provided
     order_cycle_provided = @order_cycle.present?
-    errors.add(:base, "Please choose an order cycle for this order.") unless order_cycle_provided
+    errors.add(:base, I18n.t(:spree_order_cycle_error)) unless order_cycle_provided
     order_cycle_provided
   end
 

config/locales/en.yml

@@ -2214,6 +2214,7 @@ See the %{link} to find out more about %{sitename}'s features and to start using
   spree_classification_primary_taxon_error: "Taxon %{taxon} is the primary taxon of %{product} and cannot be deleted"
   spree_order_availability_error: "Distributor or order cycle cannot supply the products in your cart"
   spree_order_populator_error: "That distributor or order cycle can't supply all the products in your cart. Please choose another."
+  spree_order_cycle_error: "Please choose an order cycle for this order."
   spree_order_populator_availability_error: "That product is not available from the chosen distributor or order cycle."
   spree_distributors_error: "At least one hub must be selected"
   spree_user_enterprise_limit_error: "^%{email} is not permitted to own any more enterprises (limit is %{enterprise_limit})."