Return org.freedesktop.DBus.Error.UnknownMethod for "No such method" error
This change makes qdbus to query property name. qdbus first tries to call method supplied by command line argument and if it fails with errororg.freedesktop.DBus.Error.UnknownMethod then it try to read property via org.freedesktop.DBus.Properties.Get method call.
Other dbus client implementations also return UnknownMethod error when method does not exist.