How to customize the message / return code Http of an expired / invalid token in Web Api 2 + Owin?

When my token expires and becomes invalid, I get a 404 (Not found) error return, but the truth is that it should be a 401 (Unauthorized). The problem is that I do not know how to customize this. I'm using Asp.Net Web Api 2 with Owin.

public class Startup {
    public void Configuration(IAppBuilder app) {

        HttpConfiguration config = new HttpConfiguration() {

            IncludeErrorDetailPolicy = IncludeErrorDetailPolicy.Always 
        };

        // Configurando injeção de dependência
        var container = new UnityContainer();
        ResolvedorDependencias.registrar(container);
        config.DependencyResolver = new UnityResolver(container);

        // habilitando o CORS
        app.UseCors(Microsoft.Owin.Cors.CorsOptions.AllowAll);

        IUsuarioServico servico = container.Resolve<IUsuarioServico>();

        OAuthAuthorizationServerOptions OAuthServerOptions = new OAuthAuthorizationServerOptions() {
            AllowInsecureHttp = true, // TODO trocar para false quando for para produção
            TokenEndpointPath = new PathString("/api/token"),
            AccessTokenExpireTimeSpan = TimeSpan.FromDays(2),
            Provider = new ProvedorDeAutorizacao(servico),
            RefreshTokenProvider = new RefreshTokenProvider(), // provê um refresh_token para recuperar um novo token quando este expirar
        };

        // Geração do token com login local
        app.UseOAuthAuthorizationServer(OAuthServerOptions);
        app.UseOAuthBearerAuthentication(new OAuthBearerAuthenticationOptions());

        WebApiConfig.Register(config);
        app.UseWebApi(config);

        ConfigureWebApi(config);
    }

    public static void ConfigureWebApi(HttpConfiguration config) {

        var formatters = config.Formatters;

        // Remove o XML
        formatters.Remove(formatters.XmlFormatter);

        var jsonSettings = formatters.JsonFormatter.SerializerSettings;

        // Modifica a identação para fins didáticos
        // TODO remover quando for para produção
        jsonSettings.Formatting = Formatting.Indented;

        // configura as propriedades para minusculo
        jsonSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();

        // tirando a referência circular
        jsonSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore; 

        // Modifica a serialização
        formatters.JsonFormatter.SerializerSettings.PreserveReferencesHandling = PreserveReferencesHandling.None;
    }
}

public class RefreshTokenProvider : AuthenticationTokenProvider {
    public override void Create(AuthenticationTokenCreateContext context) {
        // Expira em 15 dias (tempo em segundos)
        // 15 dias * 24 horas * 60 minutos * 60 segundos
        int expire = 15 * 24 * 60 * 60;
        context.Ticket.Properties.ExpiresUtc = new DateTimeOffset(DateTime.Now.AddSeconds(expire));
        context.SetToken(context.SerializeTicket());
    }

    public override void Receive(AuthenticationTokenReceiveContext context) {
        context.DeserializeTicket(context.Token);
    }
}

(I already researched the stack overflow in English and found nothing, I do not know if I'm searching wrong)