Error handling in WebClient

Question

Hello! I am building a program that keeps eye on website and notices the user if the content of the site changes. My problem is error handling... to be presice: the lack of it. If the website that program is monitoring goes down (for example error 500) my program crashes. Otherwise it is working great. How do i add error handling that makes announcement "Site is down" incase WebClient gets error 500? Here is clip of my code (names in finnish):

public void Is_Alive()
        {
            bool onko;
            string sisaltaa = "Third-party server not responding";
            string tietoteksti = "No info";
            string vahdattava = "http://www.hereisurl.com";

            WebClient myWebClient = new WebClient();
            byte[] bufferi = myWebClient.DownloadData(vahdattava);
            string tulos = Encoding.ASCII.GetString(bufferi);
            onko = tulos.Contains(sisaltaa);
            if (onko == true)
            {
                tietoteksti = "Changes!";
                SystemSounds.Exclamation.Play();
                downtimelaskuri();
                if (viimeksikinalhaalla==false)
                {
                kyykyta();
                }

                if (notifyIcon1.Visible == true)
                {
                    notifyIcon1.ShowBalloonTip(30, "Wake up!", "Changes!", ToolTipIcon.Warning);
                }

                viimeksikinalhaalla=true;
            }
            else
            {
                tietoteksti = "It's the same.";
                viimeksikinalhaalla=false;
            }
            label1.Text = tietoteksti;
        }