message: [sql lab] handle query stop race condition
  fixes
  In rare cases where the query is stopped before it is started, SQL Lab
  returns an unexpected string payload instead of a normal dictionary.
  This aligns the process to handle the error in a homogeneous fashion.
diff: @@ -232,13 +232,7 @@ def execute_sql(
  conn.close()
  if query.status == utils.QueryStatus.STOPPED:
  - return json.dumps(
  - {
  - 'query_id': query.id,
  - 'status': query.status,
  - 'query': query.to_dict(),
  - },
  - default=utils.json_iso_dttm_ser)
  + return handle_error('The query has been stopped')
  cdf = convert_results_to_df(cursor_description, data)
  