Streamlines validation response handling in controllers

This makes validation response handling more inline with aspnetcore patterns.

src/Umbraco.Web.BackOffice/Controllers/BackOfficeNotificationsController.cs

@@ -1,4 +1,8 @@
-using Umbraco.Cms.Web.BackOffice.Filters;
+using Microsoft.AspNetCore.Mvc;
+using Umbraco.Cms.Core.Models.ContentEditing;
+using Umbraco.Cms.Web.BackOffice.Filters;
+using Umbraco.Cms.Web.Common.ActionsResults;
+using Umbraco.Extensions;
 
 namespace Umbraco.Cms.Web.BackOffice.Controllers
 {
@@ -11,5 +15,30 @@ namespace Umbraco.Cms.Web.BackOffice.Controllers
     [AppendCurrentEventMessages]
     public abstract class BackOfficeNotificationsController : UmbracoAuthorizedJsonController
     {
+        /// <summary>
+        /// Overridden to ensure that the error message is an error notification message
+        /// </summary>
+        /// <param name="errorMessage"></param>
+        /// <returns></returns>
+        protected override ActionResult ValidationProblem(string errorMessage)
+        {
+            var notificationModel = new SimpleNotificationModel
+            {
+                Message = errorMessage
+            };
+            notificationModel.AddErrorNotification(errorMessage, string.Empty);
+            return new ValidationErrorResult(notificationModel);
+        }
+
+        /// <summary>
+        /// Overridden to ensure that all queued notifications are sent to the back office
+        /// </summary>
+        /// <returns></returns>
+        [NonAction]
+        public override ActionResult ValidationProblem()
+            // returning an object of INotificationModel will ensure that any pending
+            // notification messages are added to the response.
+            => new ValidationErrorResult(new SimpleNotificationModel());
+    
     }
 }