vpn: client does not show error if polkit agent not available

Before, we used to display a dialog explaining the user that we could not find a polkit agent running if we failed to find one after atempting to launch it. Now, you get "traffic is being routed in the clear" and the stopped eip icon, but the message is only displayed in the logs.

(from redmine: created on 2014-04-30, closed on 2015-03-02, relates #5595 (closed))