[FIX] point_of_sale: unmark order finalized when invoice failed

When we validate an order that need to be invoiced, and the customer is
not set, you have to enter a customer, but your order is finalized so
you have a traceback saying it cannot be modified when you try to set
the customer.

When the invoicing fails, we don't set the order as finalized.

addons/point_of_sale/static/src/js/screens.js

@@ -1916,6 +1916,7 @@ var PaymentScreenWidget = ScreenWidget.extend({
 
             invoiced.fail(function(error){
                 self.invoicing = false;
+                order.finalized = false;
                 if (error.message === 'Missing Customer') {
                     self.gui.show_popup('confirm',{
                         'title': _t('Please select the Customer'),