Refacto error parsing (HfHubHttpError)

[Wauplin]

(sorry for long PR, most of it is moving logic around) Done as part of https://github.com/huggingface/huggingface_hub/issues/2069.

This PR updates how HfHubHttpError is formatted. With https://github.com/huggingface/huggingface_hub/issues/2069, all errors are now defined in src/huggingface_hub/errors.py with no logic it the module. Before this PR, the logic to format the error message from a server response was split between HfHubHttpError.__init__ and hf_raise_for_status. This should not have being split in the first place (harder to read/investigate)

What does this PR do?

• [x] move everything from HfHubHttpError.__init__ to hf_raise_for_status. This is the only way to instantiate a HfHubHttpError anyway
• [x] move hf_raise_for_status definition inside ./utils/_http.py where it belongs instead of ./utils/_errors.py
• [x] delete ./utils/_errors.py module to avoid confusion with ./errors.py where all exceptions are defined
• [x] (fix or feature?) when an error is returned by the server as a raw payload -not json formatted-, we must add the server message to the error message. This was not the case before. It is especially useful when TGI returns a 422 unprocessable entity with explanation as raw text
• [x] keep request_id / server_message as attributes. I'm pretty sure it's not used by anyone but at least it keeps backward compatibility.
• [x] adapt tests to pass (and remove occurrences of self.assertEquals...)

In the end, sorry for the long diff because 95% of it is simply moving some logic around.

[HuggingFaceDocBuilderDev]

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[Wauplin]

As spotted by @osanseviero, this will break an import in datasets-viewer test suite @severo: https://github.com/huggingface/dataset-viewer/blob/e8e0edfb3e4b76173675d6637c170441abd42141/e2e/tests/utils.py#L15

You can already fix this by doing from huggingface_hub.utils import hf_raise_for_status.

[Wauplin]

Thanks for the review @LysandreJik! I addressed @osanseviero 's comment and resolved the merge conflicts. Failing CI is unrelated so I'm merging now.