Mirror the s3api change for the native filer HTTP handlers. When the
filer GET hits a remote-only object whose cache fill hasn't completed,
return 503 Service Unavailable with Retry-After: 5 instead of 500
Internal Error, and treat client disconnects as silent cancellations
rather than logging them as errors.
Adds an ErrCacheNotReady sentinel and a small helper used at the
prepareWriteFn-error sites in ProcessRangeRequest, so the same
classification (cancel / not-ready / other) applies to plain GETs,
single-range, and multi-range requests.
Chris Lu
578fed7d30