...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.
* {@link SuccessorsFunction#successors(Object)} can only be called for a subset of all nodes.
   */
  @Test
  public void forGraph_breadthFirst_iterableIsLazy() {
    RequestSavingGraph graph = new RequestSavingGraph(DIAMOND_GRAPH);
    Iterable<Character> result = Traverser.forGraph(graph).breadthFirst('a');
    assertEqualCharNodes(Iterables.limit(result, 2), "ab");
    assertThat(graph.requestedNodes).containsExactly('a', 'a', 'b');
    
    assertEqualCharNodes(Iterables.limit(result, 2), "ab");
    assertThat(graph.requestedNodes).containsExactly('a', 'a', 'a', 'b', 'b');
  }
  @Test
  public void forGraph_breadthFirstIterable_iterableIsLazy() {
    RequestSavingGraph graph = new RequestSavingGraph(DIAMOND_GRAPH);
    Iterable<Character> result = Traverser.forGraph(graph).breadthFirst(charactersOf("ab"));
    assertEqualCharNodes(Iterables.limit(result, 2), "ab");
    assertThat(graph.requestedNodes).containsExactly('a', 'a', 'b', 'b');
    
    assertEqualCharNodes(Iterables.limit(result, 2), "ab");
    assertThat(graph.requestedNodes).containsExactly('a', 'a', 'a', 'b', 'b', 'b');
  }
  @Test
  public void forGraph_depthFirstPreOrder_javadocExample_canBeIteratedMultipleTimes() {
    Iterable<Character> result = Traverser.forGraph(JAVADOC_GRAPH).depthFirstPreOrder('a');
    assertEqualCharNodes(result, "abecfd");
    assertEqualCharNodes(result, "abecfd");
  }
  @Test
  public void forGraph_depthFirstPreOrderIterable_javadocExample_canBeIteratedMultipleTimes() {