The purge command now explains failed deletions and interrupted runs

The purge command deletes stored files. When it could not delete something, it used to say only how many items failed. It now catches each failed deletion, records the reason, and adds: "What could not be deleted is still on disk: fix the cause and run the command again, or delete those paths by hand."

If a purge is interrupted, it now prints "Purge stopped before it finished" and suggests running it with --dry-run, which shows what would be deleted without deleting anything.