protected override void OnException(ExceptionContext filterContext)
{
    var action = filterContext.RouteData.Values["action"].ToString();
    var type = filterContext.Controller.GetType();
    var httpMethod = filterContext.RequestContext.HttpContext.Request.HttpMethod;
    filterContext.ExceptionHandled = true;

    var returnType = GetReturnType(type, action, httpMethod);
    if (returnType == typeof(JsonResult))
        filterContext.Result = HandleJsonError(filterContext.Exception);
    else
        filterContext.Result = HandleError(filterContext.Exception);
}

{"OK":false,"ErrorMessage":"Error message to the user"}