Give a better error message for private datasets
When accessing a private dataset without credentials, or with the wrong credentials, we get the same error response as for inexistent datasets, which prevent disclosing the name of private datasets:

```
{"error":"The dataset does not exist, or is not accessible without authentication (private or gated). Please check the spelling of the dataset name or retry with authentication."}
```

But when passing credentials, we get:

```
{"error":"Not found."}
```

We could give a specific message telling that the private datasets are not supported, for example. See https://github.com/huggingface/datasets-server/issues/39.

To test this, I opened https://datasets-server.huggingface.co/splits?dataset=severo/test_private_datasets in a tab, while being logged in on HF, so that the cookie is passed. This dataset is a private dataset of mine.
Give a better error message for private datasets: When accessing a private dataset without credentials, or with the wrong credentials, we get the same error response as for inexistent datasets, which prevent disclosing the name of private datasets:

```
{"error":"The dataset does not exist, or is not accessible without authentication (private or gated). Please check the spelling of the dataset name or retry with authentication."}
```

But when passing credentials, we get:

```
{"error":"Not found."}
```

We could give a specific message telling that the private datasets are not supported, for example. See https://github.com/huggingface/datasets-server/issues/39.

To test this, I opened https://datasets-server.huggingface.co/splits?dataset=severo/test_private_datasets in a tab, while being logged in on HF, so that the cookie is passed. This dataset is a private dataset of mine.