/splits does not error when no config exists and a wrong config is passed
https://datasets-preview.huggingface.tech/splits?dataset=sent_comp&config=doesnotexist

returns:

```
{

    "splits": [
        {
            "dataset": "sent_comp",
            "config": "doesnotexist",
            "split": "validation"
        },
        {
            "dataset": "sent_comp",
            "config": "doesnotexist",
            "split": "train"
        }
    ]

}
```

instead of giving an error.

As https://datasets-preview.huggingface.tech/configs?dataset=sent_comp returns

```
{
    "configs": [
        {
            "dataset": "sent_comp",
            "config": "default"
        }
    ]
}
```

the only allowed `config` parameter should be `default`.
/splits does not error when no config exists and a wrong config is passed: https://datasets-preview.huggingface.tech/splits?dataset=sent_comp&config=doesnotexist

returns:

```
{

    "splits": [
        {
            "dataset": "sent_comp",
            "config": "doesnotexist",
            "split": "validation"
        },
        {
            "dataset": "sent_comp",
            "config": "doesnotexist",
            "split": "train"
        }
    ]

}
```

instead of giving an error.

As https://datasets-preview.huggingface.tech/configs?dataset=sent_comp returns

```
{
    "configs": [
        {
            "dataset": "sent_comp",
            "config": "default"
        }
    ]
}
```

the only allowed `config` parameter should be `default`.