When remote.cache downloads a file in parallel chunks and a gRPC
connection drops mid-transfer, chunks already written to volume servers
were not cleaned up. Since the filer metadata was never updated, these
needles became orphaned — invisible to volume.vacuum and never
referenced by the filer. On subsequent cache cycles the file was still
treated as uncached, creating more orphans each attempt.
Call DeleteUncommittedChunks on the download-error path, matching the
cleanup already present for the metadata-update-failure path.
Fixes#8481
f4073107cb