Close #26: Show notification when content cannot be saved because of validation errors.

src/Squidex/app/features/content/pages/content/content-page.component.ts

@@ -137,6 +137,8 @@ export class ContentPageComponent extends AppComponentBase implements OnDestroy,
                         this.enable();
                     });
             }
+        } else {
+            this.notifyError('Content element not valid, please check the field with the red bar on the left in all languages (if localizable).');
         }
     }