`MultiWriter` recorded per-writer failures only in a private `errs` vector and
nulled the failed writer locally, but `put_object` never used that to prune the
disk set: `rename_data` ran over the full `shuffle_disks`, so a disk that took a
short/failed write still had its truncated shard renamed into place and counted
as an online disk. The object then claimed N good shards while one was
short/corrupt, so a single later disk failure could drop it below reconstructable
quorum — silent data loss.
- `write_shard`: null the writer on a generic write error too (not only on
`ShortWrite`), so a failed writer is uniformly represented as `None` — matching
`shutdown_writer`, which already does this.
- `put_object`: after encode, drop every disk whose writer failed
(`drop_failed_writer_disks`) from `shuffle_disks` before `rename_data`, and
re-check write quorum over the survivors. `rename_data` already re-checks
quorum and rolls back if too few disks remain, so excluded disks are neither
committed nor counted (MinIO sets failed writers to nil before `renameData`).
Adds unit tests for the exclusion/quorum accounting.
Refs backlog#799 (B3).
