...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_javadocExample_canBeIteratedMultipleTimes() {
    Iterable<Character> result = Traverser.forGraph(JAVADOC_GRAPH).depthFirstPostOrder('a');
    assertEqualCharNodes(result, "fcebda");
    assertEqualCharNodes(result, "fcebda");
  }
  @Test
  public void forGraph_depthFirstPostOrderIterable_javadocExample_canBeIteratedMultipleTimes() {
    Iterable<Character> result =
        Traverser.forGraph(JAVADOC_GRAPH).depthFirstPostOrder(charactersOf("bf"));
    assertEqualCharNodes(result, "efcdab");
    assertEqualCharNodes(result, "efcdab");
  }
  @Test
  public void forGraph_depthFirstPostOrder_diamond() {
    Traverser<Character> traverser = Traverser.forGraph(DIAMOND_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPostOrder('a'), "dbca");
    assertEqualCharNodes(traverser.depthFirstPostOrder('b'), "db");
    assertEqualCharNodes(traverser.depthFirstPostOrder('c'), "dc");
    assertEqualCharNodes(traverser.depthFirstPostOrder('d'), "d");
  }
  @Test
  public void forGraph_depthFirstPostOrderIterable_diamond() {
    Traverser<Character> traverser = Traverser.forGraph(DIAMOND_GRAPH);
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("")), "");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("bc")), "dbc");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("a")), "dbca");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("acdb")), "dbca");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("db")), "db");
  }