toGerror

If @message is not of type %G_DBUS_MESSAGE_TYPE_ERROR does nothing and returns false.

Otherwise this method encodes the error in @message as a #GError using g_dbus_error_set_dbus_error() using the information in the %G_DBUS_MESSAGE_HEADER_FIELD_ERROR_NAME header field of @message as well as the first string item in @message's body.