`Sink::flush` should return a `Result`, not `()`.

[blt]

Right now the flush function has no way of signaling error. As @pulltab noted the InfluxDB sink flush will loop infinitely without an influxdb to report to, which is... not great. #353 is caused by an inability to signal error upward.

This can be repaired by having Result kicked out of the function, or some other kind of failure.