/**
  * If no param, returns last error
  * If param is exception, logs exception and sets error
  * If param is String, sets error
  *
  * @static
  * @param null $err
  * @return mixed
  */
 public static function error($err = null)
 {
     if ($err == null) {
         return CertificateManager::$error;
     }
     if ($err instanceof Exception) {
         CertificateManager::$error = $err->getMessage();
         LoggerInterface::logException($err, LoggerInterface::ERROR);
     } else {
         CertificateManager::$error = $err;
         LoggerInterface::log($err, LoggerInterface::ERROR);
     }
 }