...in favor of just calling `setException` unconditionally.

I wrote in a code review:

> There's a decent argument that the work we do here and in MoreExecutors.rejectionPropagatingExecutor isn't really buying us much, since it's not clear where else a RejectedExecutionException would be coming from. And even if it is coming from somewhere else, the most plausible location of all the implausible locations is "somewhere after we already called set/setException/setFuture/cancel," in which case we could still call setException(), confident in the knowledge that it will be a no-op.
> 
> Or maybe I'm forgetting something important. Beats me :) But certainly it's hard (impossible?) to write a test that covers this case.
private static class ListeningDecorator extends AbstractListeningExecutorService {
    private final ExecutorService delegate;
    ListeningDecorator(ExecutorService delegate) {
      this.delegate = checkNotNull(delegate);
    }
    @Override
    public final boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
      return delegate.awaitTermination(timeout, unit);