Catch Java exception from Runnable in Unit test

I have the below code

public final class JoinableTaskPool<T> extends ABC {
       private int taskCounter;
       private final Object monitor;
       private final ExtendedThreadPoolExecutor service;
       private final CompletionService<T> compService;

       public Future<T> submit(final Callable<T> task) {
           final Future<T> result = compService.submit(task);
           service.submit(new Runnable() {
             public void run() {
                try {
                    final Future<T> result = compService.take();
                    try {
                        handler.processResult(result);
                    } catch (final Throwable t) {
                        throw new SearchException("Task has an error", t);
                    }
                } catch (InterruptedException e) {
                    throw new SearchException("Task has an error", e);
                }
            }
          } 
          return result;
       }

  public void join() {
    synchronized (monitor) {
        while (taskCounter != 0) {
            try {
                monitor.wait();
            } catch (InterruptedException e) {
                error(e, "Interrupted in join");
            }
        }
  }
  }

The ExtendedThreadPoolExecutor class is defined as follows

public class ExtendedThreadPoolExecutor extends ThreadPoolExecutor {

    public ExtendedThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);
    }

    @Override
    protected void afterExecute(Runnable r, Throwable t) {
        super.afterExecute(r, t);
        if(t != null) {
            throw new SearchException("Error while executing the task", t);
        }
    }
}

I am trying to write a unit test for this method. Below is the method

@RunWith(MockitoJUnitRunner.class)
public class TestJoinableTaskPool<T> {

    private JoinableTaskPool<T> pool;

    @Before
    public void setUp() {
        pool = new JoinableTaskPool<T>(1);
    }

    @After
    public void tearDown() {
        pool.shutdown();
    }

    @Test (expected = SearchException.class)
    public void testSubmit() throws Exception{
        Callable task = (Callable<T>) () -> null;

        Mockito.when(pool.getFinishHandler().processResult(result))
                .thenThrow(RuntimeException.class);

        pool.submit(task);
    }
}

Since the SearchException exception is thrown in runnable, there is no way to access it outside the submit method. If I would have returned the Future returned by executorService.submit, I could have done a future.get() to get an exception, but I am returning another future (result variable). So while writing the unit test I am not able to get the exception thrown.

Also I have overriden the afterExecute() in an effort to catch exception from unit test,but couldnt find a way to call it.

How do I test this exception thrown from the runnable from a unit test. Any help would be appreciated.