Fetch failures are currently tracked byy AsyncSearchTask like ordinary shard failures. Though they should be treated differently or they end up causing weird scenarios like total=num_shards and successful=num_shards as the query phase ran fine yet the failed count would reflect the number of shards where fetch failed.
Given that partial results only include aggs for now and are complete even if fetch fails, we can ignore fetch failures in async search, as they will be anyways included in the response. They are in fact either received as a failure when all shards fail during fetch, or as part of the final response when only some shards fail during fetch.
b680d3fb29