classic ASP.NET Custom error page not displaying error message

I have created a cusom error page in ASP.NET 4.6 to display error message.

I get the exception details as below and store in a session variable in Application_Error section of Global.asax.cs

Exception exception = HttpContext.Current.Server.GetLastError().GetBaseException();

The page displays error message when called from Global.asax.cs but when I redeirect from web.config with custom error mode as on, error message is not displayed in the page.

Please advise.