Extend BulkFailureException.failedDocuments return type to show more details about failure

[ilya-ulyanov]

Whenever a bulk operation (insert or update) fails, there maybe a need to handle a failure of every document in a specific way based on a reason of failure.

Relying on a String error message coming from native Elasticsearch library (the current implementation) is not reliable, as requires checking for a specific sub-string, and the message itself may change in the future.

This PR proposes a change, which will expose an HTTP response code (taken from https://artifacts.elastic.co/javadoc/co/elastic/clients/elasticsearch-java/7.17.0/co/elastic/clients/elasticsearch/core/bulk/BulkResponseItem.html#status()) in addition to currently exposed error message, so that it will be much more convenient to handle specific failure reasons (e.g. 409 - conflict, when trying to save a document with version less than or equal to the currently stored in Elasticsearch).

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Closes #2619