Return Elasticsearch warnings only in res.warnings

[pquentin]

Closes #33

I was not able to test this on a real cluster as:

• elasticsearch-js does not expose warnings
• I did not figure out how to use the transport client directly

But a way to test this is to run a ... 7.17 cluster and close an index:

curl -v -X PUT http://localhost:9200/test-index/
curl -v -X POST http://localhost:9200/test-index/_close

Or to run a 8.12 cluster and use GET _cat/aliases?local=true (https://github.com/elastic/elasticsearch/issues/101815).