shell: Immediately close superuser dialog when cancelling
shell: Immediately close superuser dialog when cancelling

Sudo sometimes takes a bit to terminate, but having the dialog stay
open is confusing.  People might think that they have clicked the
"Authenticate" button by accident, maybe.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=2073263

martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit