...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.
* }</pre>
   *
   * <p>See <a href="https://en.wikipedia.org/wiki/Breadth-first_search">Wikipedia</a> for more
   * info.
   *
   * @throws IllegalArgumentException if {@code startNode} is not an element of the graph
   */
  public final Iterable<N> breadthFirst(N startNode) {
    return breadthFirst(ImmutableSet.of(startNode));
  }
  /**
   * Returns an unmodifiable {@code Iterable} over the nodes reachable from any of the {@code
   * startNodes}, in the order of a breadth-first traversal. This is equivalent to a breadth-first
   * traversal of a graph with an additional root node whose successors are the listed {@code
   * startNodes}.
   *
   * @throws IllegalArgumentException if any of {@code startNodes} is not an element of the graph
   * @see #breadthFirst(Object)
   * @since 24.1
   */
  public final Iterable<N> breadthFirst(Iterable<? extends N> startNodes) {
    final ImmutableSet<N> validated = validate(startNodes);
    return new Iterable<N>() {
      @Override
      public Iterator<N> iterator() {
        return newTraversal().breadthFirst(validated.iterator());
      }
    };
  }
  /**