Support for bot error HTTP status code

Direct Line now returns an httpStatusCode field in the error JSON accompanying HTTP 502s caused by bot timeouts/errors. Is it possible to wire this up so clients have an easy time seeing the bot's HTTP status code?

Example below illustrates a bot returning HTTP 404.

HTTP/1.1 502
...

{
  "error": {
    "code": "ServiceError",
    "message": "Failed to send activity: bot returned an error"
  },
  "httpStatusCode": 404
}

首先定位 Direct Line 的错误响应处理，并检查 JSON 错误负载如何暴露给客户端。使用提供的 502 响应（包括 httpStatusCode 404）作为预期情况；当客户端可以访问该 bot HTTP 状态且 coverage 验证了这一点时，即表示完成。