error through ErrorHandling.

Hi i need to display my error in message box.

i have caught my error through ErrorHandling.

How should i specify The error in The label in a message box.


 
catch
{

lblError.Text = Request.QueryString["Err"];

string error = lblError.Text;
Response.Write("window.alert(error)");

}


This does not display the message box
 
Thanks 
...