promql: Holt-Winters benchmark does not detect failures.

[beorn7]

From #1743 :
"However, while doing that, I also noticed that the Holt Winters benchmarks actually fail, but that the error return value is ignored (https://github.com/prometheus/prometheus/blob/master/promql/bench.go#L42) :( I guess that's one way of tuning benchmarks :) I'll remark that on #1485 to see how to proceed."

This needs to be fixed. @eliothedeman would be great if you could provide context or even a fix.

[lock]

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