GET /{bucket}/: optional override to return partial results when a server is down

Follow-up to #41.

#41 made `GET /{bucket}/` on the locator return a clean `503` when any object server is unreachable or errors — but it still fails the whole request if even one server is down.

This issue tracks the deferred second half of #41: an optional override (e.g. a `?partial=true` query parameter) that, instead of `503`, returns the merged listing from the object servers that **are** up, skipping the ones that are down.

Design questions to settle:
- Parameter name (`?partial=true` / `?available=true` / `?best_effort=true`).
- How to signal partiality in the response — the current body is `{"bucket": ..., "objects": ...}`; silently returning an incomplete listing is risky, so it should probably flag that results are partial and/or list the unavailable servers.
- Whether the override should also apply to `HEAD /{bucket}/`.