ModelStateValidationAttribute.cs

    public class ModelStateValidationAttribute : ActionFilterAttribute, IExceptionFilter
    {
        public override void OnActionExecuting(ActionExecutingContext filterContext) {
            if(filterContext.HttpContext.Request.IsAjaxRequest())
                return;

            var modelIsValid = filterContext.Controller.ViewData.ModelState.IsValid;

            if(!modelIsValid) {
                filterContext.Result = new ViewResult() {
                    ViewData = filterContext.Controller.ViewData
                };
            }
        }

        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.Result = new ViewResult {
                ViewData = filterContext.Controller.ViewData,
                TempData = filterContext.Controller.TempData
            };

            filterContext.ExceptionHandled = true;
        }
    }