Bad message when config name is missing
When loading a dataset that have several configurations, we expect to see an error message if the user doesn't specify a config name.

However in `datasets` 1.10.0 and 1.10.1 it doesn't show the right message:

```python
import datasets

datasets.load_dataset("glue")
```
raises
```python
AttributeError: 'BuilderConfig' object has no attribute 'text_features'
```
instead of
```python
ValueError: Config name is missing.
Please pick one among the available configs: ['cola', 'sst2', 'mrpc', 'qqp', 'stsb', 'mnli', 'mnli_mismatched', 'mnli_matched', 'qnli', 'rte', 'wnli', 'ax']
Example of usage:
        `load_dataset('glue', 'cola')`
```