ListingFailure

The loader could not enumerate its documents: an S3 listing that failed (no credentials, missing bucket, access denied), a directory that does not exist.

Unlike Failure, this is not about one document. The set of documents the loader would have produced is unknown, so it cannot be counted as one failed document among successes: RAG.ingest, RAG.sync and RAG.refresh (and their async forms) return its error as a Left, and sync deletes nothing, rather than treat every indexed document as gone. A listing that fails part-way through (a later S3 page) is reported the same way, after the documents that were listed before it.