message: slack api - extend error object with response details if rate limit
diff: @@ -280,7 +280,9 @@ module.exports = function(bot, config) {
  return cb((json.ok ? null : json.error), json);
  } else if (response.statusCode == 429) {
  - return cb(new Error('Rate limit exceeded'));
  + var rateLimitError = new Error('Rate limit exceeded');
  + rateLimitError.response = response;
  + return cb(rateLimitError);
  } else {
  return cb(new Error('Invalid response'));
  }
  