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
}

[watadarkstar]

I’ll try my best! 💪 I still don’t fully understand what the retry id is telling me half the time, but I’ll propose a PR with what I think would be a better solution.

[watadarkstar]

In our app we have a fork that has something like this to get better error information: https://github.com/Microsoft/BotFramework-DirectLineJS/pull/115