Display error on email (triggered by advance_order_state method)

app/controllers/split_checkout_controller.rb

@@ -37,9 +37,9 @@ class SplitCheckoutController < ::BaseController
     load_shipping_method
     handle_shipping_method_selection
 
-    if confirm_order || update_order
+    confirm_or_update = confirm_order || update_order
+    if confirm_or_update && advance_order_state
       clear_invalid_payments
-      advance_order_state
       redirect_to_step
     else
       if @shipping_method_id.blank?

app/models/spree/order.rb

@@ -501,7 +501,7 @@ module Spree
     def disallow_guest_order
       return unless using_guest_checkout? && registered_email?
 
-      errors.add(:base, I18n.t('devise.failure.already_registered'))
+      errors.add(:email, I18n.t('devise.failure.already_registered'))
     end
 
     # After changing line items of a completed order

spec/models/spree/order_spec.rb

@@ -1059,7 +1059,7 @@ describe Spree::Order do
 
     it "returns a validation error" do
       expect{ order.next }.to change(order.errors, :count).from(0).to(1)
-      expect(order.errors.messages[:base]).to eq [I18n.t('devise.failure.already_registered')]
+      expect(order.errors.messages[:email]).to eq [I18n.t('devise.failure.already_registered')]
       expect(order.state).to eq 'cart'
     end
   end