Expose the ratelimit error detail from the response

[stevenxi]

As an API consumer itself, respect Slack's Rate Limits (https://api.slack.com/docs/rate-limits) may be difficult as SlackAPI. But it would be nice to expose the rate limit error detail in the response.

Currently the response (LoginResponse as sample), only provides error message as "ratelimit" but doesn't provide how long the client need to wait before next request (Retry-After).

So the consumer of SlackAPI client do not have a clue about how long it should wait.

Thanks.