Always send disconnected() on socket errors

Some socket errors (e.g. connection refused) don't trigger a disconnected()
from the socket, mostly because the socket hadn't been opened in the
first place.

To simplify the logic elsewhere, let's always send (exactly) one disconnected()
from the AuthHandler in case of socket errors. That way, we don't have to
introduce the mess we used to have in the reconnection logic in CoreConnection.