Shut down the SC2 process.

def quit(self):
    """Shut down the SC2 process."""
    try:
      # Don't expect a response.
      self._client.write(sc_pb.Request(quit=sc_pb.RequestQuit()))
    except protocol.ConnectionError:
      pass  # It's likely already (shutting) down, so continue as if it worked.
    finally:
      self.close()