When API request fails, log message for better troubleshooting

还没有人认领这个 Issue。

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

从 DirectionsResponse 模型开始，跟踪当前在响应体为 null 时如何处理 API 失败。确认失败请求中的 `message` 值应如何暴露，以及是否应记录日志或引发运行时异常。当诸如 `Overview option must be full for congestion` 之类的失败不再被静默处理时，即视为完成。

由索引模型根据 Issue 内容生成。