Make sharded checkpoints work in offline mode
# What does this PR do?

This PR make sharded checkpoint work in offline mode and add more information to an error we return.
The crux of the issue is that the `from_pretrained` method of the various models will catch `EntryNotFoundError` on the regular model weights file, but we return a `FileNotFoundError` in offline mode. I changed the error type at the root, to avoid making three modifications in the PyTorch/TF/Flax model classes, but can change if you don't find this suitable.
_The documentation is not available anymore as the PR was closed or merged._