...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.
public void forTree_depthFirstPostOrder_graphContainingTreeAndDiamond() throws Exception {
    Traverser<Character> traverser = Traverser.forTree(GRAPH_CONTAINING_TREE_AND_DIAMOND);
    assertEqualCharNodes(traverser.depthFirstPostOrder('a'), "cdba");
    assertEqualCharNodes(traverser.depthFirstPostOrder('b'), "cdb");
    assertEqualCharNodes(traverser.depthFirstPostOrder('d'), "d");
  }
  @Test
  public void forTree_depthFirstPostOrderIterable_graphContainingTreeAndDiamond() throws Exception {
    Traverser<Character> traverser = Traverser.forTree(GRAPH_CONTAINING_TREE_AND_DIAMOND);
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("a")), "cdba");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("bg")), "cdbhg");
    assertEqualCharNodes(traverser.depthFirstPostOrder(charactersOf("ga")), "hgcdba");
  }
  @Test
  public void forTree_depthFirstPostOrder_twoTrees() {
    Iterable<Character> result = Traverser.forTree(TWO_TREES).depthFirstPostOrder('a');
    assertEqualCharNodes(result, "ba");
  }
  @Test
  public void forTree_depthFirstPostOrderIterable_twoTrees() {
    assertEqualCharNodes(Traverser.forTree(TWO_TREES).depthFirstPostOrder(charactersOf("a")), "ba");
    assertEqualCharNodes(
        Traverser.forTree(TWO_TREES).depthFirstPostOrder(charactersOf("ca")), "dcba");
  }
  @Test
  public void forTree_depthFirstPostOrder_singleRoot() {
    Iterable<Character> result = Traverser.forTree(SINGLE_ROOT).depthFirstPostOrder('a');
    assertEqualCharNodes(result, "a");
  }
  @Test