Return the executed notebook.

def get_executed_notebook(self):
        """Return the executed notebook."""
        r = requests.get('http://{0}/api/notebook/{1}'.format(
                         self.zeppelin_url, self.notebook_id))
        if r.status_code == 200:
            return r.json()['body']
        else:
            print('ERROR: Could not get executed notebook.', file=sys.stderr)
            sys.exit(1)