Fix error handling when creating a voucher adjustment

I wrongly assumed that `voucher.create_adjustment` would return nil
if failing to create an adjustment. I will in fact return an
adjustment object with errors.

app/controllers/split_checkout_controller.rb

@@ -226,8 +226,9 @@ class SplitCheckoutController < ::BaseController
 
     adjustment = voucher.create_adjustment(voucher.code, @order)
 
-    if adjustment.nil?
+    if !adjustment.valid?
       @order.errors.add(:voucher, I18n.t('split_checkout.errors.add_voucher_error'))
+      adjustment.errors.each { |error| @order.errors.import(error) }
       return false
     end