Refine errors for pretrained objects
# What does this PR do?

This PR breaks refines the error returned for pretrained objects using the error codes now returned by hf.co to clearly indicate to the user if:
- there is a problem in the repo name
- there is a problem in the revision
- there is a problem with a specific file
- there is a general connection error

Whenever we are in a situation where the model has a version in another framework, a helpful error message is shown.

Also note that this PR removes the hard failure on `get_list_of_files` whenever there is a connection problem, which should make the CI way less flaky.
_The documentation is not available anymore as the PR was closed or merged._