[AutoFeatureExtractor] Fix loading of local folders if config.json exists
# What does this PR do?

Currently there is a problem when loading the feature extractor locally via `AutoFeatureExtractor` as spotted by @philschmid:

```bash
!git lfs install
!git clone https://huggingface.co/facebook/wav2vec2-base-960h
```

and then:

```python
from transformers import AutoFeatureExtractor

extractor = AutoFeatureExtractor.from_pretrained("wav2vec2-base-960h")
```

This leads to an error. This PR fixes it and also improves the error message.
@sgugger - merging for now. Let me know if something is not well and we can change afterwards. Tests have been added so this PR should be more or less safe.