...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.
Iterable<Character> result =
        Traverser.forGraph(JAVADOC_GRAPH).depthFirstPreOrder(charactersOf("bc"));
    assertEqualCharNodes(result, "bacefd");
    assertEqualCharNodes(result, "bacefd");
  }
  @Test
  public void forGraph_depthFirstPreOrder_infinite() {
    Iterable<Integer> result =
        Traverser.forGraph(fixedSuccessors(Iterables.cycle(1, 2, 3))).breadthFirst(0);
    assertThat(Iterables.limit(result, 2)).containsExactly(0, 1).inOrder();
  }
  @Test
  public void forGraph_depthFirstPreOrder_diamond() {
    Traverser<Character> traverser = Traverser.forGraph(DIAMOND_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPreOrder('a'), "abdc");
    assertEqualCharNodes(traverser.depthFirstPreOrder('b'), "bd");
    assertEqualCharNodes(traverser.depthFirstPreOrder('c'), "cd");
    assertEqualCharNodes(traverser.depthFirstPreOrder('d'), "d");
  }
  @Test
  public void forGraph_depthFirstPreOrderIterable_diamond() {
    Traverser<Character> traverser = Traverser.forGraph(DIAMOND_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPreOrder(charactersOf("")), "");
    assertEqualCharNodes(traverser.depthFirstPreOrder(charactersOf("bc")), "bdc");
    assertEqualCharNodes(traverser.depthFirstPreOrder(charactersOf("a")), "abdc");
    assertEqualCharNodes(traverser.depthFirstPreOrder(charactersOf("acdb")), "abdc");
    assertEqualCharNodes(traverser.depthFirstPreOrder(charactersOf("db")), "db");
  }
  @Test
  public void forGraph_depthFirstPreOrder_multigraph() {