Format: the `failed` entries in aggregated steps should have more information
See https://github.com/huggingface/datasets-server/issues/1338

```json
{
  "parquet_files": [],
  "pending": [],
  "failed": [
    {
      "kind": "config-parquet",
      "dataset": "bigcode/the-stack-dedup",
      "config": "bigcode--the-stack-dedup",
      "split": null
    }
  ]
}
```

returned by https://datasets-server.huggingface.co/parquet?dataset=bigcode%2Fthe-stack-dedup misses information about why the steps have failed.

We should return the error, or at least the error_code
Format: the `failed` entries in aggregated steps should have more information: See https://github.com/huggingface/datasets-server/issues/1338

```json
{
  "parquet_files": [],
  "pending": [],
  "failed": [
    {
      "kind": "config-parquet",
      "dataset": "bigcode/the-stack-dedup",
      "config": "bigcode--the-stack-dedup",
      "split": null
    }
  ]
}
```

returned by https://datasets-server.huggingface.co/parquet?dataset=bigcode%2Fthe-stack-dedup misses information about why the steps have failed.

We should return the error, or at least the error_code