avoid noisy context-cancelled error on shutdown

[aybabtme]

^C
{"time":"2025-04-03T00:28:33.801818+09:00","level":"ERROR","source":{"function":"github.com/humanlogio/humanlog/pkg/sink/logsvcsink.(*ConnectStreamSink).connectAndHandleBuffer.func3","file":"github.com/humanlogio/humanlog/pkg/sink/logsvcsink/stream_sink.go","line":146},"msg":"closing and receiving response for log ingestor session","sink":"local","machine_id":12606,"err":"canceled: Post \"http://localhost:32764/svc.ingest.v1.IngestService/IngestStream\": context canceled"}
{"time":"2025-04-03T00:28:34.105798+09:00","level":"ERROR","source":{"function":"main.newApp.func16.5","file":"github.com/humanlogio/humanlog/cmd/humanlog/main.go","line":697},"msg":"couldn't flush buffered log (localhost)","err":"context deadline exceeded"}

this is annoying