Include slice_id for failed slices in reindex task response

[DylanGriffith]

When tracking a Reindex using the task ID I get the following examples for slices in the response:

        {
          "type": "search_phase_execution_exception",
          "reason": "Phase failed",
          "phase": "fetch",
          "grouped": true,
          "failed_shards": [],
          "caused_by": {
            "type": "node_not_connected_exception",
            "reason": "[instance-0000000054][123.123.123.123:1234] Node not connected"
          }
        },
        {
          "slice_id": 38,
          "total": 4077527,
          "updated": 0,
          "created": 10000,
          "deleted": 0,
          "batches": 10,
          "version_conflicts": 0,
          "noops": 0,
          "retries": {
            "bulk": 0,
            "search": 0
          },
          "throttled_millis": 0,
          "requests_per_second": -1,
          "throttled_until_millis": 0
        },

From this I can assume that slice_id: 37 was the one that failed because the all appear in order but it would be nicer to consume this data if the failed slice also had an ID.

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Reindex)