Make Transformers use cache files when hf.co is down
# What does this PR do?

This PR makes sure that we actually go look in the cached files when hf.co is down (or the internet connection of the user). The main issue was that we were raising the `HTTPError` too fast and didn't execute the code path that goes look in the cached files.

Since that error is now delayed, in case of a connection error and file not in the cache, we end up in the `ValueError` raised by `get_from_cache` [here](https://github.com/huggingface/transformers/blob/12428f0ef15bb3631e7a5f04672ddb05f363de97/src/transformers/utils/hub.py#L538), so I had to adapt a bit the cascade of errors in the `from_pretrained` methods.

This is then tested for all objects with a `from_pretrained` method (except Flax models since I didn't find a tiny flax model).
_The documentation is not available anymore as the PR was closed or merged._