Improve download error messages

[GregoryComer]

Improve CLI error messages when user has not logged in via huggingface-cli login or needs to request access to a model through HuggingFace. Also add common issues section to README.

When missing the CLI login, output looks like this:

Downloading mistralai/Mistral-7B-Instruct-v0.2 from HuggingFace...
Access denied. Create a HuggingFace account and run 'pip3 install huggingface_hub' and 'huggingface-cli login' to authenticate.

When the user needs to request access to the model, output looks like this:

Downloading mistralai/Mistral-7B-Instruct-v0.2 from HuggingFace...
403 Client Error. (Request ID: Root=1-6629a4c2-4c457124529cc04b0164f042;d1e1e6a2-8c8c-4e24-8147-8144fd97cfd1)

Cannot access gated repo for url https://huggingface.co/api/models/mistralai/Mistral-7B-Instruct-v0.2/revision/main.
Access to model mistralai/Mistral-7B-Instruct-v0.2 is restricted and you are not in the authorized list. Visit https://huggingface.co/mistralai/Mistral-7B-Instruct-v0.2 to ask for access.