Return Error instead of Discarding Data about fluency HOT 4 CLOSED

@obokaiwele FYI: Fluency 1.4.0 supports custom error handler and you can detect a send error with that.

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AsyncFlusher runs in background thread. So there is no straight way to return an error of flushing data to the user application. Instead introducing an error handler that has a callback function would work. With it, the user application can pass a custom error handler to Fluency and Fluency call the callback function of the error handler to notify a send error to the user application. I'll take care of the feature later.

RE: the backup configuration of Fluency, it is just to save buffers in memory to file when Fluency is closing not for failures of flushing data.

BTW, Fluentd being down should be detected by a monitoring system not by a Fluentd logger for robustness, I think.

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I'm going to support custom error handlers on #66

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@komamitsu Is there a way to get the payload that could not be emitted while handling the error?
My use case is to try to emit via Fluency, if that fails, then log the same data to system journald.

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