Redirecting To Custom Error Page

I have inherited the an asp.net app. So i want to add the custom Error Handling. I know there are two methods i can do this.

1) Web Config
2)Global.asa

So i started with the web config just to see how it will look and i accessed the page that normally gives an Exception. so basically i have a page gives a

500 - Internal server error.
There is a problem with the resource you are looking for, and it cannot be displayed.

So now , if there is an Error i want it to go to that Error Page. In the web config i have the Following

Code:
<customErrors defaultRedirect="content/ErrorPages/SupportError.aspx" mode="On">
<error statusCode="404" redirect="content/ErrorPages/SupportError.aspx" />
<error statusCode="400" redirect="content/ErrorPages/SupportError.aspx" />
<error statusCode="500" redirect="content/ErrorPages/SupportError.aspx" />
</customErrors>

This means that when this kind of Errors occur it should redirect to that page. All these pages are contents of the Master page including the Error page. But this Error Persist it does not show me the Error page. Even if i try to see if it goes to "Application_Error" in the Global.asa nothing goes there.

So i have also tried the second option. so i turned the web config part "off" and i trapped this in the global.asa like this

Code:
Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
Server.Transfer("~/Content/ErrorPages/SupportError.aspx")
'Response.Redirect("~/Content/ErrorPages/SupportError.aspx", False)
End Sub

But still nothing happens ...