Fix hang in qnam when disconnecting

Generate error for network requests when connection gets disconnected.
Documentation states that QNAM requests will fail if network is not
accessible, so we need to track session state.

Task-number: QTBUG-47482
Change-Id: I2c2d348637f72b2a908b438a66aa543a878de1e5
Reviewed-by: Timo Jyrinki <timo.jyrinki@canonical.com>
Reviewed-by: Richard J. Moore <rich@kde.org>