Closing cockpit-bridge-connector.pyz shows a "reconnect" button which won't work

[jelly]

If for whatever reason cockpit-bridge-connector.pyz dies, cockpit will detect this and show a special page:

The reconnect option will however not work, maybe we should just remove it?

[martinpitt]

This means that our multiplexer.py does not properly handle closing of the target machine websocket, like it does for the browser websocket? Let's fix that, and also move the session state to "closing" then, so that our own "the session has ended page appears instead?