TopicExists and getTopicConfFromCache negative-cached a topic for the full
30s TTL whenever a filer lookup failed for any reason, including timeouts.
A topic created earlier then looked gone until the TTL expired, and the
metadata auto-create path couldn't heal it (CreateTopic rejects an
already-persisted conf), so producers saw UNKNOWN_TOPIC_OR_PARTITION.
Only negative-cache on a definitive ErrNotFound; let transient errors fall
through and retry against the filer.