More informative download error messages

[rasbt]

This adds more informative error messages for the various access token issues to address #1363.

Also, I tried to reduce the stack trace @lantiga . Instead of

Traceback (most recent call last):
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/huggingface_hub/utils/_errors.py", line 304, in hf_raise_for_status
    response.raise_for_status()
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/requests/models.py", line 1021, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 401 Client Error: Unauthorized for url: https://huggingface.co/api/models/meta-llama/llama-2-7b

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 125, in gated_repo_catcher
    yield
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 115, in find_weight_files
    info = repo_info(repo_id, token=access_token)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/huggingface_hub/utils/_validators.py", line 119, in _inner_fn
    return fn(*args, **kwargs)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/huggingface_hub/hf_api.py", line 2418, in repo_info
    return method(
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/huggingface_hub/utils/_validators.py", line 119, in _inner_fn
    return fn(*args, **kwargs)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/huggingface_hub/hf_api.py", line 2228, in model_info
    hf_raise_for_status(r)
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/huggingface_hub/utils/_errors.py", line 321, in hf_raise_for_status
    raise GatedRepoError(message, response) from e
huggingface_hub.utils._errors.GatedRepoError: 401 Client Error. (Request ID: Root=1-663001a8-2fe0554741634ef222a41c49;b05217eb-e0a2-4270-9e65-d894b9c30f92)

Cannot access gated repo for url https://huggingface.co/api/models/meta-llama/llama-2-7b.
Access to model meta-llama/Llama-2-7b is restricted. You must be authenticated to access it.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/zeus/miniconda3/envs/cloudspace/bin/litgpt", line 8, in <module>
    sys.exit(main())
  File "/teamspace/studios/this_studio/litgpt/litgpt/__main__.py", line 143, in main
    fn(**kwargs)
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 54, in download_from_hub
    bins, safetensors = find_weight_files(repo_id, access_token)
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 114, in find_weight_files
    with gated_repo_catcher(repo_id, access_token):
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 141, in gated_repo_catcher
    raise ValueError(
ValueError: https://huggingface.co/meta-llama/llama-2-7b requires authentication. The access token provided by `HF_TOKEN=your_token` environment variable or `--access_token=your_token` may not have sufficient access rights. Please visit https://huggingface.co/meta-llama/llama-2-7b for more information.

It's now just

Traceback (most recent call last):
  File "/home/zeus/miniconda3/envs/cloudspace/bin/litgpt", line 8, in <module>
    sys.exit(main())
  File "/teamspace/studios/this_studio/litgpt/litgpt/__main__.py", line 143, in main
    fn(**kwargs)
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 54, in download_from_hub
    bins, safetensors = find_weight_files(repo_id, access_token)
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 114, in find_weight_files
    with gated_repo_catcher(repo_id, access_token):
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/teamspace/studios/this_studio/litgpt/litgpt/scripts/download.py", line 141, in gated_repo_catcher
    raise ValueError(
ValueError: https://huggingface.co/meta-llama/llama-2-7b requires authentication. The access token provided by `HF_TOKEN=your_token` environment variable or `--access_token=your_token` may not have sufficient access rights. Please visit https://huggingface.co/meta-llama/llama-2-7b for more information.

If we want to not raise a ValueError at all, we could print the message and use quit() for example. But I remember that @carmocca had some arguments against it which is why we prefer raising errors.

Fixes #1363

[carmocca]

LGTM.

Exceptions are very useful for those who call our code outside of the CLI as it lets them catch it and reason about them. They are also useful when there's an actual underlying issue (in our code or the code of our dependencies). But if It's preferable to hide the exceptions for product reasons, then let's do it.