detect if there is a vpn instance already running
When unexpected errors happen, ui crashes and a child process (run as root!) is still running. Use psutil for this, we are likely to need it for related tasks and it's nice to have a multiplatform lib ready for those.
We should message the user for manual intervention. (since we cannot kill this instance ourselves).
Related tickets following this deal with shutting it down, and securing the management interface using a nonce.
Add a _check_if_running_instance(self) method to openvpnconnection.OpenVPNConnection, and raise a new eip.exceptions that inherits from CriticalError (it will make the dialog automatically, showing what's in usermessage attribute).
(from redmine: created on 2012-09-20, closed on 2012-10-08, relates #607 (closed), relates #679 (closed), relates #2916 (closed))