Download NApp package from server.

        Return:
            str: Downloaded temp filename.

        Raises:
            urllib.error.HTTPError: If download is not successful.

def _download(self):
        """Download NApp package from server.

        Return:
            str: Downloaded temp filename.

        Raises:
            urllib.error.HTTPError: If download is not successful.

        """
        repo = self._config.get('napps', 'repo')
        napp_id = '{}/{}-{}.napp'.format(self.user, self.napp, self.version)
        uri = os.path.join(repo, napp_id)
        return urllib.request.urlretrieve(uri)[0]