...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 abstract class CombinedFutureInterruptibleTask<T> extends InterruptibleTask<T> {
    private final Executor listenerExecutor;
    CombinedFutureInterruptibleTask(Executor listenerExecutor) {
      this.listenerExecutor = checkNotNull(listenerExecutor);
    }
    @Override
    final boolean isDone() {
      return CombinedFuture.this.isDone();
    }
    final void execute() {
      try {
        listenerExecutor.execute(this);
      } catch (RejectedExecutionException e) {
        CombinedFuture.this.setException(e);
      }