Add error message if adding a voucher fails

It's not really useful to the user but better than failing silently

app/controllers/split_checkout_controller.rb

@@ -203,9 +203,12 @@ class SplitCheckoutController < ::BaseController
       return false
     end
 
-    # Create adjustment
-    # TODO add tax part of adjustement
-    voucher.create_adjustment(voucher.code, @order)
+    adjustment = voucher.create_adjustment(voucher.code, @order)
+
+    if adjustment.nil?
+      @order.errors.add(:voucher, I18n.t('split_checkout.errors.add_voucher_error'))
+      return false
+    end
 
     true
   end

config/locales/en.yml

@@ -2095,6 +2095,7 @@ en:
       select_a_payment_method: Select a payment method
       no_shipping_methods_available: Checkout is not possible due to absence of shipping options. Please contact the shop owner.
       voucher_not_found: Not found
+      add_voucher_error: There was an error while adding the voucher
   order_paid: PAID
   order_not_paid: NOT PAID
   order_total: Total order

spec/controllers/split_checkout_controller_spec.rb

@@ -279,7 +279,16 @@ describe SplitCheckoutController, type: :controller do
           end
 
           context "when adding fails" do
-            pending  "returns 422 and an error message"
+            it "returns 422 and an error message" do
+              # Makes adding the voucher fails
+              allow(voucher).to receive(:compute_amount).and_return(0)
+              allow(Voucher).to receive(:find_by).and_return(voucher)
+
+              put :update, params: params
+
+              expect(response.status).to eq 422
+              expect(flash[:error]).to match "There was an error while adding the voucher"
+            end
           end
         end