IndexError when loading ImageFolder
## Describe the bug
Loading an image dataset with `imagefolder` throws `IndexError: list index out of range` when the given folder contains a non-image file (like a csv).

## Steps to reproduce the bug
Put a csv file in a folder with images and load it:
```python
import datasets
 datasets.load_dataset("imagefolder", data_dir=path/to/folder)
```

## Expected results
I would expect a better error message, like `Unsupported file` or even the dataset loader just ignoring every file that is not an image in that case.

## Actual results
Here is the whole traceback:

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.3.2
- Platform: Linux-5.11.0-051100-generic-x86_64-with-glibc2.27
- Python version: 3.9.9
- PyArrow version: 8.0.0
- Pandas version: 1.4.3
