return BadRequest(error: "کاربر مورد نظر یافت نشد");

<form
...
...
data-ajax-failure="dataAjaxFailure">

function dataAjaxFailure(xhr, status, error) {
    alert(xhr.responseText);
}

var f = db.Faq.Where(x=>x.IdHamayesh == id && x.IdLanguage == idLanguage).OrderBy(x=>x.Order).Cacheable().ToList();

[RegularExpression("^[a-zA-Z0-9_]*$", ErrorMessage = "لطفا تنها از اعداد و حروف انگلیسی استفاده نمائید")]
public string Username { get; set; }

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Abstractions;
using Microsoft.AspNetCore.Mvc.ActionConstraints;
using Microsoft.AspNetCore.Routing;

namespace WebToolkit
{
    public static class AjaxExtensions
    {
        private const string RequestedWithHeader = "X-Requested-With";
        private const string XmlHttpRequest = "XMLHttpRequest";

        public static bool IsAjaxRequest(this HttpRequest request)
        {
            return request?.Headers != null && request.Headers[RequestedWithHeader] == XmlHttpRequest;
        }
    }

    public class AjaxOnlyAttribute : ActionMethodSelectorAttribute
    {
        public override bool IsValidForRequest(RouteContext routeContext, ActionDescriptor action)
        {
            return routeContext.HttpContext.Request.IsAjaxRequest();
        }
    }
}