When a message is received from Gstreamer.

def _on_message(self, bus, message):  # pylint: disable=unused-argument
        """When a message is received from Gstreamer."""
        if message.type == Gst.MessageType.EOS:
            self.stop()
        elif message.type == Gst.MessageType.ERROR:
            self.stop()
            err, _ = message.parse_error()
            _LOGGER.error('%s', err)