Use MVC's model state on Ajax forms, provided that there's some client-side code to capture the validation output

ValidateAjaxAttribute.cs

    /// <summary>
    /// http://stackoverflow.com/questions/14005773/use-asp-net-mvc-validation-with-jquery-ajax
    /// </summary>
    public class ValidateAjaxAttribute : ActionFilterAttribute, IExceptionFilter
    {
        public override void OnActionExecuting(ActionExecutingContext filterContext) {
            if(!filterContext.HttpContext.Request.IsAjaxRequest()) {
                return;
            }

            var modelState = filterContext.Controller.ViewData.ModelState;
            if (modelState.IsValid) {
                return;
            }

            filterContext.HttpContext.Response.StatusCode = (int)HttpStatusCode.BadRequest;

            filterContext.Result = GetModelErrorData(modelState);
            
        }

        public override void OnActionExecuted(ActionExecutedContext filterContext) {
            if(!filterContext.HttpContext.Request.IsAjaxRequest()) {
                return;
            }

            var modelState = filterContext.Controller.ViewData.ModelState;
            if (modelState.IsValid) {
                return;
            }

            filterContext.HttpContext.Response.StatusCode = (int)HttpStatusCode.BadRequest;

            filterContext.Result = GetModelErrorData(modelState);
            
        }

        public void OnException(ExceptionContext filterContext) {
            if (!filterContext.HttpContext.Request.IsAjaxRequest())
                return;

            var modelState = filterContext.Controller.ViewData.ModelState;

            var exception = filterContext.Exception as CustomException;
            if (exception != null) {
                modelState.AddModelError(exception.PropertyName, exception.Message);
            }
            else {
                modelState.AddModelError(string.Empty, Resources.APPLICATION_ERROR);
            }

            filterContext.HttpContext.Response.StatusCode = (int)HttpStatusCode.BadRequest;

            filterContext.Result = GetModelErrorData(modelState);

            filterContext.ExceptionHandled = true;
        }

        private static JsonNetResult GetModelErrorData(IDictionary<string, ModelState> modelState) {
            
            var errorModel = 
                from x in modelState.Keys
                where modelState[x].Errors.Count > 0
                select new {
                    key = x,
                    errors = modelState[x].Errors.
                        Select(y => y.ErrorMessage).
                        ToArray()
                };

            return new JsonNetResult(new JsonSerializerSettings() { Formatting = Formatting.Indented }) {
                Data = errorModel
            };
        }

        
    }