[FIX] point_of_sale: display python except message on partner creation

Having base_vat installed (but is potentially more general than this)

In the pos frontend, try to save a partner with an incorrect VAT

Before this commit, the generic and cryptic message "your internet is probably down" was displayed to the user
when a python exception occured

After this commit, we display the exception's message

OPW 807656

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

@@ -1177,9 +1177,14 @@ var ClientListScreenWidget = ScreenWidget.extend({
             self.saved_client_details(partner_id);
         },function(err,event){
             event.preventDefault();
+            var error_body = _t('Your Internet connection is probably down.');
+            if (err.data) {
+                var except = err.data;
+                error_body = except.arguments && except.arguments[0] || except.message || error_body;
+            }
             self.gui.show_popup('error',{
                 'title': _t('Error: Could not Save Changes'),
-                'body': _t('Your Internet connection is probably down.'),
+                'body': error_body,
             });
         });
     },