discard any exception thrown while trying to kill worker threads

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.