...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.
* @param terminationTimeout how long to wait for the executor to finish before terminating the
   *     JVM
   * @param timeUnit unit of time for the time parameter
   * @return an unmodifiable version of the input which will not hang the JVM
   */
  @Beta
  @GwtIncompatible 
  @SuppressWarnings("GoodTime") 
  public static ExecutorService getExitingExecutorService(
      ThreadPoolExecutor executor, long terminationTimeout, TimeUnit timeUnit) {
    return new Application().getExitingExecutorService(executor, terminationTimeout, timeUnit);
  }
  /**
   * Converts the given ThreadPoolExecutor into an ExecutorService that exits when the application
   * is complete. It does so by using daemon threads and adding a shutdown hook to wait for their
   * completion.
   *
   * <p>This method waits 120 seconds before continuing with JVM termination, even if the executor
   * has not finished its work.
   *
   * <p>This is mainly for fixed thread pools. See {@link Executors#newFixedThreadPool(int)}.
   *
   * @param executor the executor to modify to make sure it exits when the application is finished
   * @return an unmodifiable version of the input which will not hang the JVM
   */
  @Beta
  @GwtIncompatible 
  public static ExecutorService getExitingExecutorService(ThreadPoolExecutor executor) {
    return new Application().getExitingExecutorService(executor);
  }