The DBUS API actually doesn't support the subcall feature.
Until the subcall is implemented, it is better to emit an error
than to fall back to the default behaviour (calling in the context
of the service).
Change-Id: Ibf5f3524995692a509d2d648d61e9ed0f874d7c1 Signed-off-by: José Bollo <jose.bollo@iot.bzh>