...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.
/**
 * An object that can traverse the nodes that are reachable from a specified (set of) start node(s)
 * using a specified {@link SuccessorsFunction}.
 *
 * <p>There are two entry points for creating a {@code Traverser}: {@link
 * #forTree(SuccessorsFunction)} and {@link #forGraph(SuccessorsFunction)}. You should choose one
 * based on your answers to the following questions:
 *
 * <ol>
 *   <li>Is there only one path to any node that's reachable from any start node? (If so, the graph
 *       to be traversed is a tree or forest even if it is a subgraph of a graph which is neither.)
 *   <li>Are the node objects' implementations of {@code equals()}/{@code hashCode()} <a
 *       href="https://github.com/google/guava/wiki/GraphsExplained#non-recursiveness">recursive</a>?
 * </ol>
 *
 * <p>If your answers are:
 *
 * <ul>
 *   <li>(1) "no" and (2) "no", use {@link #forGraph(SuccessorsFunction)}.
 *   <li>(1) "yes" and (2) "yes", use {@link #forTree(SuccessorsFunction)}.
 *   <li>(1) "yes" and (2) "no", you can use either, but {@code forTree()} will be more efficient.
 *   <li>(1) "no" and (2) "yes", <b><i>neither will work</i></b>, but if you transform your node
 *       objects into a non-recursive form, you can use {@code forGraph()}.
 * </ul>
 *
 * @author Jens Nyman
 * @param <N> Node parameter type
 * @since 23.1
 */
@Beta