Backup silently drops extended attributes it cannot read #596

LocalSourceSaveOptions::map_entry (backend/ignore/mapper.rs, 0.13.0 and current main) catches a failure from xattrs(), logs "ignoring error obtaining xargs" and yields the entry with no extended attributes. The snapshot then records the entry as having none, and nothing reaches the caller: #561's RusticProgress::error and error_count do not cover it.

xattrs() also fails as a whole when one xattr::get fails, so one unreadable attribute discards the rest.

Repro (macOS):

echo hi > f; xattr -w com.example.k v f
chmod +a "everyone deny readextattr" f
# back up f: content stored, attributes empty, no error

Wanted: report the failure like other source errors (e.g. through RusticProgress::error during archival and error_count), so a caller can tell "no attributes" from "attributes not read".