Throw an exception when hitting the rate limit (429) instead of generic exception

When we are rate limited the client just throws a generic exception. It would great if it could throw a specific exception [here](https://github.com/elastic/site-search-ruby/blob/e434d93b576379d93903c0045d378f4bb5f6bb5e/lib/elastic/site-search/request.rb#L91-L116) and also expose the rate limit headers so we can do something about it.

```
X-RateLimit-Limit: xxxx
X-RateLimit-Remaining: xxxx
```