Cherry picks ValidateUmbracoFormRouteStringAttribute implementation and fixes up some logic

src/Umbraco.Web/Mvc/HttpUmbracoFormRouteStringException.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Net;
+using System.Web;
+
+namespace Umbraco.Web.Mvc
+{
+    /// <summary>
+    /// Exception that occurs when an Umbraco form route string is invalid
+    /// </summary>
+    /// <seealso cref="System.Web.HttpException" />
+    [Serializable]
+    public sealed class HttpUmbracoFormRouteStringException : HttpException
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="HttpUmbracoFormRouteStringException" /> class.
+        /// </summary>
+        /// <param name="message">The error message displayed to the client when the exception is thrown.</param>
+        public HttpUmbracoFormRouteStringException(string message)
+            : base(message)
+        { }
+
+    }
+}