function formatMessage(opt, desc) {
    return '[Axios v' + VERSION + '] Transitional option \'' + opt + '\'' + desc + (message ? '. ' + message : '');
  }
Transitional option validator
@param {function|boolean?} validator - set to false if the transitional option has been removed
@param {string?} version - deprecated version / removed since version
@param {string?} message - some message with additional info
@returns {function}