docs: public boolean exists(String indexName) {
          try {
              final JestResult result = jestClient.execute(new GetSettings.Builder().addIndex(indexName).build());
              return result.isSucceeded() && Iterators.contains(result.getJsonObject().fieldNames(), indexName);
          } catch (IOException e) {
              throw new ElasticsearchException("Couldn't check existence of index " + indexName, e);
          }
      }