When API request fails, log message for better troubleshooting

Just spent some time figuring out why my response body was null only to learn that the request was failing with a message Overview option must be full for congestion. We should add a field for message in the DirectionsResponse for example and if that message isn't null, throw a runtime exception or at least log it.

cc: @zugaldia

從 DirectionsResponse 模型開始，追蹤目前在回應本文為 null 時如何處理 API 失敗。確認失敗請求中的 message 值應如何公開，以及是否應記錄日誌或引發執行階段例外。當諸如 Overview option must be full for congestion 之類的失敗不再被靜默處理時，即視為完成。