protected function getErrorMessage($errorCode)
    {
        if (!isset($this->errorMessageMatrix[$errorCode])) {
            $this->errorMessageMatrix[$errorCode] = 'ERROR_MSG_UNKNOWN';
        }

        // if additional information exist -> add it here
        if ($this->info) {
            if (is_string($this->info)) {
                $this->errorMessageMatrix[$errorCode] .= ' '.$this->info;
            } elseif (count($this->info) != 1 || $this->info[0] != null) {
                foreach ($this->info as $info) {
                    $info = serialize($info);
                    $this->errorMessageMatrix[$errorCode] .= ' '.$info;
                }
            }
        }

        // Return constructed error message
        return $this->errorMessageMatrix[$errorCode];
    }