[FIX] website_sale_wishlist: manage warning in return value

Similar to https://github.com/odoo/odoo/blob/11.0/addons/website_sale/static/src/js/website_sale.js#L173

It is particulary interesing when website_sale_stock return warning.
Until now, the end user had no info about the failed add_to_cart

addons/website_sale/controllers/main.py

@@ -400,9 +400,10 @@ class WebsiteSale(http.Controller):
             return {}
 
         value = order._cart_update(product_id=product_id, line_id=line_id, add_qty=add_qty, set_qty=set_qty)
+
         if not order.cart_quantity:
             request.website.sale_reset()
-            return {}
+            return value
 
         order = request.website.sale_get_order()
         value['cart_quantity'] = order.cart_quantity

addons/website_sale_stock/models/sale_order.py

@@ -25,9 +25,10 @@ class SaleOrder(models.Model):
                     # Make sure line still exists, it may have been deleted in super()_cartupdate because qty can be <= 0
                     if line.exists() and new_val['quantity']:
                         line.warning_stock = _('You ask for %s products but only %s is available') % (cart_qty, new_val['quantity'])
+                        values['warning'] = line.warning_stock
                     else:
                         self.warning_stock = _("Some products became unavailable and your cart has been updated. We're sorry for the inconvenience.")
-
+                        values['warning'] = self.warning_stock
         return values
 
     @api.multi

addons/website_sale_wishlist/static/src/js/website_sale_wishlist.js

@@ -134,7 +134,14 @@ var ProductWishlist = Widget.extend({
         });
 
         add_to_cart.then(function(resp) {
-            $('.my_cart_quantity').text(resp.cart_quantity || '* 1');
+            if (resp.warning) {
+                if (! $('#data_warning').length) {
+                    $('.wishlist-section').prepend('<div class="mt16 alert alert-danger alert-dismissable" role="alert" id="data_warning"></div>');
+                }
+                var cart_alert = $('.wishlist-section').parent().find('#data_warning');
+                cart_alert.html('<button type="button" class="close" data-dismiss="alert" aria-hidden="true">&times;</button> ' + resp.warning);
+            }
+            $('.my_cart_quantity').html(resp.cart_quantity || '<i class="fa fa-warning" /> ');
         });
         return add_to_cart;
     },