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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def upload_site(self):
 """Upload a previously-built site to LSST the Docs."""

        if not os.path.isdir(self._config['build_dir']):
            message = 'Site not built at {0}'.format(self._config['build_dir'])
            self._logger.error(message)
            raise RuntimeError(message)

        ltdclient.upload(self._config)