return 404 for piece retrievals correctly

[LexLuthr]

No description provided.

[juliangruber]

Closes #797

[Kubuxu]

There are more places in curio when the multi-error wrapping with xerrors happens.

market/storageingest/deal_ingest_snap.go
417:				return nil, nil, xerrors.Errorf("checking sector status: %w, %w, %w", err1, err2, err3)

web/api/webrpc/market.go
209:			return &StorageDealSummary{}, xerrors.Errorf("failed to parse the deal ID: %w and %w", err, perr)

tasks/seal/task_synth_proofs.go
127:				return xerrors.Errorf("store %s failure: updating pipeline: Original error %w: DB error %w", name, err, serr)