...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.
*/
  public static <N> Traverser<N> forGraph(final SuccessorsFunction<N> graph) {
    return new Traverser<N>(graph) {
      @Override
      Traversal<N> newTraversal() {
        return Traversal.inGraph(graph);
      }
    };
  }
  /**
   * Creates a new traverser for a directed acyclic graph that has at most one path from the start
   * node(s) to any node reachable from the start node(s), and has no paths from any start node to
   * any other start node, such as a tree or forest.
   *
   * <p>{@code forTree()} is especially useful (versus {@code forGraph()}) in cases where the data
   * structure being traversed is, in addition to being a tree/forest, also defined <a
   * href="https://github.com/google/guava/wiki/GraphsExplained#non-recursiveness">recursively</a>.
   * This is because the {@code forTree()}-based implementations don't keep track of visited nodes,
   * and therefore don't need to call `equals()` or `hashCode()` on the node objects; this saves
   * both time and space versus traversing the same graph using {@code forGraph()}.
   *
   * <p>Providing a graph to be traversed for which there is more than one path from the start
   * node(s) to any node may lead to:
   *
   * <ul>
   *   <li>Traversal not terminating (if the graph has cycles)
   *   <li>Nodes being visited multiple times (if multiple paths exist from any start node to any
   *       node reachable from any start node)
   * </ul>
   *