...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 final class CallableInterruptibleTask extends CombinedFutureInterruptibleTask<V> {
    private final Callable<V> callable;
    CallableInterruptibleTask(Callable<V> callable, Executor listenerExecutor) {
      super(listenerExecutor);
      this.callable = checkNotNull(callable);
    }
    @Override
    V runInterruptibly() throws Exception {
      return callable.call();
    }
    @Override
    void setValue(V value) {
      CombinedFuture.this.set(value);
    }
    @Override
    String toPendingString() {
      return callable.toString();
    }
  }
}