connect: log async errors and lame-duck mode through the connection logger

Without an ErrorHandler, nats.go routes async errors — slow-consumer drops, permissions violations on missing server grants — to a default handler that prints to stderr, bypassing the service's structured logs. Every service that noticed (mirror-pipeline, at six Connect sites) attached its own handler via WithNATSOptions; make it the default posture instead. A lame-duck warn log likewise attributes a server-maintenance eviction to maintenance rather than a fault. Both handlers remain overridable via WithNATSOptions, which applies after the defaults.