Gets notification msgs on the server side to be displayed. Fixes property existence checking... was checking on id before but that is incorrect, needs to be on alias since a new property id is gen'd on each save and we're not re-wiring them back up. Fixes YSOD messaging for content as well. Changes the ModelState to modelState to be consistent in js.

src/Umbraco.Web.UI.Client/test/unit/common/services/content-editing-helper.spec.js

@@ -22,7 +22,7 @@ describe('contentEditingHelper tests', function () {
                 data: content,
                 status: 403
             };
-            err.data.ModelState = {};
+            err.data.modelState = {};
 
             //act
             var handled = contentEditingHelper.handleSaveError(err, {content: content});