SO migrations heavily rely on indexing, searching, and writing data to Elasticsearch. Any request to Elasticsearch may fail. The current implementation of SO migration does not show the error reason provided by the Elasticsarch server in logs. Which makes it's nearly impossible to debug failures that originated in the Elasticsearch server.
Given: a search request with a closed PIT.
actual error message in logs: ResponseError: illegal_argument_exception
expected error message in logs: ResponseError. root_cause: [{ type: 'illegal_argument_exception', reason: '...' }]

We might consider fixing the serialization format in @elastic/elasticsearch to cover all the use-cases. elastic/elasticsearch-js#1452