Context.Response.StatusCode = 403;

        protected void Application_PreSendRequestContent(object sender, EventArgs e) { 
            if (Context.Response.StatusCode == 302)
            {
                Context.Response.ClearContent();
                Context.Response.ClearHeaders();
                //значение 600 - за рамками стандарта и является значением определенным пользователем
                Context.Response.StatusCode = 600;
                string data = @"{success: false, data: [{redirectUrl: 'index.html', Message: 'Session expired or not authorized request'}]}";
                Context.Response.Write(data);
                Context.Response.End();
            }
        }

Ext.util.Observable.observe(Ext.data.Connection);
Ext.Ajax.on('requestexception', function (dataconn, response, options) {
    if (response.responseText != null) {
        var resp = Ext.decode(response.responseText);
        Ext.Msg.show({
            title: 'Ошибка',
            msg: resp.data[0].Message,
            buttons: Ext.Msg.OK,
            icon: Ext.window.MessageBox.INFO
        });
    }
});

 public class FaultErrorHandler : IErrorHandler {

        public bool HandleError(Exception error) {
            return true;
        }
        public void ProvideFault(Exception error, MessageVersion version, ref Message fault) {
            // extract the our FaultContract object from the exception object.
            //var detail = FaultMessage.CreateFaultMessage(error);
            var detail = new ExceptionDetail(error);
            // create a fault message containing our FaultContract object
            fault = Message.CreateMessage(version, "", detail, new DataContractJsonSerializer(typeof(ExceptionDetail)));
            // tell WCF to use JSON encoding rather than default XML
            var wbf = new WebBodyFormatMessageProperty(WebContentFormat.Json);
            fault.Properties.Add(WebBodyFormatMessageProperty.Name, wbf);
            HttpResponseMessageProperty property;
            property = new HttpResponseMessageProperty();
            fault.Properties.Add(HttpResponseMessageProperty.Name, property);
            property.Headers.Add("jsonerror", "true");
            property.StatusCode = System.Net.HttpStatusCode.InternalServerError;

        }
    }

[DataContract]
    public class FaultMessage {
        [DataMember(Name = "success")]
        public Boolean Success = false;
        [DataMember(Name = "data")]
        public IList<ExceptionDetail> Data { private set; get; }
        private FaultMessage() {
            this.Data = new List<ExceptionDetail>();
        }
        public static FaultMessage CreateFaultMessage(Exception exception) {
            FaultMessage result = new FaultMessage();
            result.Data.Add(new ExceptionDetail(exception));
            return result;
        }
    }

    [DataContract]
    [Serializable]
    public class ExceptionDetail {
        public ExceptionDetail(Exception exception) {
            if (exception.InnerException!=null) {
                exception = exception.InnerException;
            }
            this.Message = exception.Message;
            this.Type = exception.GetType().ToString();
        }
        [DataMember(Order = 1)]
        public string Message { get; private set; }
        [DataMember(Order = 2)]
        public string Type { get; private set; }
    }