Error Handling 404 in Asp.Net MVC 5

ObjectNotFoundExceptionFilterAttribute.cs

using System.Web.Mvc;

namespace YourWebsite.Filters
{
    public class ObjectNotFoundExceptionFilterAttribute : ActionFilterAttribute, IExceptionFilter
    {
        public void OnException(ExceptionContext filterContext)
        {
            if (filterContext.Exception is NHibernate.ObjectNotFoundException)
            {
                filterContext.Result = new HttpNotFoundResult("Entity not found");
                filterContext.ExceptionHandled = true;
            }
        }
    }
}

WebConfig.Snippet

...
 <system.web>
    <customErrors mode="On" redirectMode="ResponseRewrite" defaultRedirect="/Error">
      <error statusCode="404" redirect="NotFound"/>
    </customErrors>
  <!-- other stuff here-- >
  </system.web>
 
  <system.webServer>
    <httpErrors errorMode="Custom" existingResponse="Replace">
      <remove statusCode="404" />
      <error statusCode="404" responseMode="ExecuteURL" path="/NotFound" />
    </httpErrors>
  <!-- other stuff here-- >
  </system.webServer>
...