...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.
@Test
  public void forGraph_depthFirstPostOrder_multigraph() {
    Traverser<Character> traverser = Traverser.forGraph(MULTI_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPostOrder('a'), "dbca");
    assertEqualCharNodes(traverser.depthFirstPostOrder('b'), "db");
    assertEqualCharNodes(traverser.depthFirstPostOrder('c'), "dbac");
    assertEqualCharNodes(traverser.depthFirstPostOrder('d'), "d");
  }
  @Test
  public void forGraph_depthFirstPostOrderIterable_multigraph() {
    Traverser<Character> traverser = Traverser.forGraph(MULTI_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("ac")), "dbca");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("cb")), "dbac");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("db")), "db");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("d")), "d");
  }
  @Test
  public void forGraph_depthFirstPostOrder_cycle() {
    Traverser<Character> traverser = Traverser.forGraph(CYCLE_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPostOrder('a'), "dcba");
    assertEqualCharNodes(traverser.depthFirstPostOrder('b'), "adcb");
    assertEqualCharNodes(traverser.depthFirstPostOrder('c'), "badc");
    assertEqualCharNodes(traverser.depthFirstPostOrder('d'), "cbad");
  }
  @Test
  public void forGraph_depthFirstPostOrderIterable_cycle() {
    Traverser<Character> traverser = Traverser.forGraph(CYCLE_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("a")), "dcba");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("bd")), "adcb");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("dc")), "cbad");