...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.
valueGraph.putEdgeValue("a", "b", 11);
    try {
      Traverser.forTree(valueGraph);
      fail("Expected exception");
    } catch (IllegalArgumentException expected) {
    }
  }
  @Test
  @SuppressWarnings("CheckReturnValue")
  public void forTree_acceptsDirectedNetwork() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.directed().build();
    network.addEdge("a", "b", 11);
    Traverser.forTree(network); 
  }
  @Test
  public void forTree_withUndirectedNetwork_throws() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.undirected().build();
    network.addEdge("a", "b", 11);
    try {
      Traverser.forTree(network);
      fail("Expected exception");
    } catch (IllegalArgumentException expected) {
    }
  }
  @Test
  public void forTree_breadthFirst_infinite() {
    Iterable<Integer> result =
        Traverser.forTree(fixedSuccessors(Iterables.cycle(1, 2, 3))).breadthFirst(0);
    assertThat(Iterables.limit(result, 8)).containsExactly(0, 1, 2, 3, 1, 2, 3, 1).inOrder();
  }