Hi,
currently you are re-directing to fd.aspx destination path if being checked for fd.aspx.
For page not found error redirection, try this.
<customErrors mode="RemoteOnly" defaultRedirect="~/GeneralError.aspx">
<error statusCode="404" redirect="~/PageNotFound.aspx"/>
</customErrors>
For all other errors, it redirects to general error page.
For 404 error it redirects to pagenotfound.aspx page.
hope this helps.
Happy coding!