if (!model.Item.Path.StartsWith("http:"))

public partial class ItemController : Controller
{

		[HttpGet]
		public virtual ActionResult Index(int id)
		{
			...
			throw new HttpException(404, "Товар не найден");
			
			
			if (!model.Item.Path.StartsWith("http:"))	//здесь уже происходит ошибка
			...
		}
...
}



//класс для обработки ошибок
	[HandleError]
	public partial class ErrorController : Controller
	{
		public virtual ActionResult Index(Exception data)
		{
			string viewName = null;
			int statusCode = 500;

			if (data != null && data is HttpException)
				statusCode = (data as HttpException).GetHttpCode();

			switch (statusCode)
			{
				case 404:
					viewName = MVC.Error.Views._404;
					break;
				case 500:
					viewName = MVC.Error.Views._500;
					break;
				default:
					viewName = MVC.Error.Views.General;
					break;
			}
			Response.StatusCode = statusCode;
			return View(viewName, data);
		}

...
	}

	
	//кусок класса global.asax.cs
	
	public static void RegisterRoutes(RouteCollection routes)
	{
		routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
		routes.IgnoreRoute("favicon.ico");
		#region Catalog/Best/New/Soon

		routes.MapRoute(
				"Error - 404",
				"NotFound",
				new { controller = "Error", action = "NotFound" }
		);
		...
	}

	//web.config
	
	...
		<customErrors mode="On" redirectMode="ResponseRedirect" defaultRedirect="Error.aspx">
			<error statusCode="403" redirect="403.aspx"/>
			<error statusCode="404" redirect="404.aspx" />
		</customErrors>
	...

throw new HttpException(404, "Товар не найден");

try{..}catch{..} 

	routes.MapRoute(
				"checkPage",
				"item/{id}",
				new { controller = "Error", action = "Index", id = UrlParameter.Optional }