discard any exception thrown while trying to kill worker threads

Since there's a race here, and since Kyle saw an exception leak out,
which I have not been able to reproduce that. See my comment for what
I think might be going on.

Note that, I used tryNonAsync, because it seems a later tryNonAsync
caught the exception. I don't actually understand how it did, as I
understand exception classification, it's the data type, not the way it
was thrown. One possibility is that the async exception may have been wrapped
in some other, non-async exception, and Show displayed it the same way.

Utility/Metered.hs

@@ -328,7 +328,7 @@ outputFilter cmd params environ outfilter errfilter =
 		-- maximum for them to finish and then cancel them.
 		-- (One program that has behaved this way in the past is
 		-- openssh.)
-		race_
+		void $ tryNonAsync $ race_
 			(wait outt >> wait errt)
 			(threadDelaySeconds (Seconds 2) >> cancel outt >> cancel errt)
 		return ret