Raise an issue when no row can be fetched
Currently, https://datasets-preview.huggingface.tech/rows?dataset=superb&config=asr&split=train&rows=5 returns

```
{
    "dataset": "superb",
    "config": "asr",
    "split": "train",
    "rows": [ ]
}
```

while it should return 5 rows. An error should be raised in that case.

Beware: manage the special case when the query parameter `rows` is greater than the number of rows in the split. In that case, it's normal that the number of returned rows is lower than requested.
Raise an issue when no row can be fetched: Currently, https://datasets-preview.huggingface.tech/rows?dataset=superb&config=asr&split=train&rows=5 returns

```
{
    "dataset": "superb",
    "config": "asr",
    "split": "train",
    "rows": [ ]
}
```

while it should return 5 rows. An error should be raised in that case.

Beware: manage the special case when the query parameter `rows` is greater than the number of rows in the split. In that case, it's normal that the number of returned rows is lower than requested.