GET /{bucket}/: optional override to return partial results when a server is down #63

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}/.