want to show "Unknown error" message to user rather than " A potentially dangerous Request.Form value was detected from the client" on page

Question

I try to prompt "Unknown error" message to user rather than " A potentially dangerous Request.Form value was detected from the client" on page where user put script tags in textbox or textarea. I have tried clientScript or scriptmanager to prompt error message to page but it does not work out. please suggest any method for show error message on page to UI side. I have tried this code as follows:

void Application_Error(object sender, EventArgs e) {

        Exception exc = Server.GetLastError();
        if (object.ReferenceEquals(exc.GetType(), typeof(HttpRequestValidationException)))
        {
            if (((System.Web.HttpRequestValidationException)exc).Message.Contains("A potentially dangerous Request.Form value was detected from the client"))
            {
                if (HttpContext.Current != null)
                {
                    var url = HttpContext.Current.Request.Url;
                    //var page = HttpContext.Current.Handler as System.Web.UI.Page;

                    Page page = HttpContext.Current.CurrentHandler as Page;
                    if (page is Page)
                    {
                        // page.ClientScript.RegisterStartupScript(GetType(), "Error","<script>alert('Unknown error')</script>");
                        Response.Redirect(url.ToString());
                    }
                }
            }
        }

    }

I also read setting for XSS as A potentially dangerous Request.Form value was detected from the client thanks for advance. please help