Application_OnError Handler, take care of JS expected responses.

Global.asax.cs

		protected void Application_Error(object sender, EventArgs e)
		{
			var status = 500;

			// Get the error details
			var lastError = Server.GetLastError();
			HttpException lastErrorWrapper = Server.GetLastError() as HttpException;

			if (lastErrorWrapper != null)
			{
				status = lastErrorWrapper.ErrorCode;
				lastError = lastErrorWrapper.InnerException ?? lastError;
			}

			string lastErrorTypeName = lastError.GetType().FullName;
			string lastErrorMessage = lastError.Message;
			string lastErrorStackTrace = lastError.StackTrace;

			// Filter logging for request-level errors
			if (lastError is System.ArgumentException)
			{
				if (lastErrorMessage.IndexOf("parameters dictionary") >= 0)
				{
					status = 400; //invalid input parameter - MVC
				}
			}

			//server-side error - log it
			if (status >= 500)
			{
				// TODO: Log Error Message Here
			}

			//set the response status to match the set status code
			if (status >= 100 || status < 600) Response.StatusCode = status;
		
			//return as JSON for appropriate requests	
			var isJson = Request.AcceptTypes.Where(at => at.IndexOf("json") >= 0).Count() > 0;
			var isJS = Request.AcceptTypes.Where(at => at.IndexOf("javascript") >= 0).Count() > 0;
			var isJsonP = !string.IsNullOrWhiteSpace(Request.QueryString["callback"]);
			if (isJsonP || isJson || isJS) {
				var data = JS.StringifyFormatted(new
				{
					error = new
					{
						code = Response.StatusCode,
						message = lastError.Message,
						details = JS.ConvertError(lastError)
					}
				});

				if (isJsonP)
				{
					//JsonP request, wrap callback
					Response.ContentType = "application/javascript";
					Response.Write(string.Format("{0}({1})", Request.QueryString["callback"], data));
				}
				else if (isJson)
				{
					//Json request, send as-is
					Response.ContentType = "application/json";
					Response.Write(data);
				}
				else if (isJS)
				{
					//JavaScript request - wrap with alert.
					Response.ContentType = "application/javascript";
					Response.Write(string.Format("alert('Unhandled server error.');console.error('Server Error:',{0});", data));
                }

				Server.ClearError(); //handled with JS response
			}
		}