Error strings added, if there are any, to Elasticsearch entries

Having error strings available in Elasticsearch, along with default data generated for servers where data collection failed, can help identify problems in the collection of X-Road environmental monitoring data.

Migration from the Elasticsearch TransportClient to the Java High Level REST Client

Using the REST interface enables communication with Elasticsearch version 7 and should be more or less version agnostic. Furthermore, this allows using the Amazon Elasticsearch Service, which does not support the TCP transport.