When http client cannot connect, provide a meaningful error

[Yoav Tock]

This code in the session.go getNodesCerts function fails to parse the response, and supply it in the error:

response, err := httpClient.Do(req)
	if err != nil {
		d.logger.Errorf("failed to send transaction to server %s, due to %s", getStatus.String(), err)
		return nil, err
	}