Expose snapshot_path on IncompleteSnapshotError

[moon-bot-app]

Should help huggingface/diffusers#14118 (see huggingface/diffusers#14118 (comment))

IncompleteSnapshotError now carries a snapshot_path attribute pointing to the (incomplete) local snapshot, so downstream libraries can still locate the latest cached files even when they are known to be incomplete. snapshot_download populates it at the raise site, the CLI appends the path to its error output when available, and the cache-management docs mention the attribute.

CLI example:

✗ HF_HUB_OFFLINE=1 hf download openai-community/gpt2
Error: Local entry not found. Cannot reach https://huggingface.co/api/models/openai-community/gpt2/revision/main: offline mode is enabled. To disable it, please unset the `HF_HUB_OFFLINE` environment variable.
Incomplete snapshot available at: /home/wauplin/.cache/huggingface/hub/models--openai-community--gpt2/snapshots/607a30d783dfa663caf39e06633721c8d4cfcd7e
Hint: set HF_DEBUG=1 as environment variable for full traceback.

and in Python:

try:
    snapshot_path = snapshot_download(...)
except IncompleteSnapshotError as error:
    incomplete_snapshot_path = error.snapshot_path

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Requested by @Wauplin - Slack thread - Agent trace

---

Note

Low Risk
Small, backward-compatible extension to an existing error type with a single internal raise site updated; no auth or download logic changes.

Overview
IncompleteSnapshotError now includes a snapshot_path attribute so callers can find partially cached files when offline or local_files_only blocks completing a download.

snapshot_download sets snapshot_path to the incomplete snapshot directory (base_dir) when raising from _raise_if_incomplete_snapshot. The hf CLI registers a dedicated formatter that appends that path to the error message (before the generic LocalEntryNotFoundError handler). Cache docs note the new attribute.

^{Reviewed by Cursor Bugbot for commit 4a48a1f. Bugbot is set up for automated code reviews on this repo. Configure here.}

[bot-ci-comment]

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[huggingface-hub-bot]

This PR has been shipped as part of the v1.23.0 release.