At this time `bufferedwriter` [silently ignores connection close
errors](78eaa056c0/lib/bufferedwriter/bufferedwriter.go (L67)).
It may be very convenient in some situations (to not log such
unimportant errors), but it's too implicit and unsafe for the others.
For example, if you close [export
API](https://docs.victoriametrics.com/#how-to-export-time-series) client
connection in the middle of communication, VictoriaMetrics won't notice
it and will start to hog CPU by exporting all the data into nowhere
until it process all of them. If you'll make a few retries, it will be
effectively a DoS on the server.
This commit replaces this implicit error suppressing with explicit error
handling which fixes the issue with export API.
Issue was introduced at e78f3ac8ac
