Improve error message of ImageFolder

[Freed-Wu]

Feature

https://github.com/InterDigitalInc/CompressAI//blob/master/compressai/datasets/image.py#L64

Why not change it to

raise RuntimeError(f'Non-existed directory "{splitdir}"')

[fracape]

sorry for the late reply, I skipped this one. Yes, that works, why not. Since this "custom" dataset expects train/valid/test subfolders, that's what needs to be suggested