public function getError(?int $i=null): string
  {
    if ( null === $i ){
      $error = end($this->errors);
    }
    else {
      if ( !array_key_exists($i, $this->errors) ){
        return false;
      }
      
      $error = $this->errors[$i];
    }
    
    if ( $error instanceof Exception ){
      return $error->getMessage();
    }
    
    return $error;
  }