Message when product no longer is avileble

When I go to a product that is deleted I'll be redirected to the first page. I want to show a page that the product no longer is avileble. How do I do that?

web.config
<customErrors defaultRedirect="errorpage.htm" mode="RemoteOnly">
      <error statusCode="404" redirect="filenotfound.htm" />
    </customErrors>