Concrete type of exception being returned from AsyncRestTemplate if server is down?

I am using AsyncRestTemplate as my HttpClient to execute my URL. And I am using exchange method of AsyncRestTemplate which returns back a ListenableFuture on which I am adding a callback to see whether the response was successful or not.

Below is my code:

private final AsyncRestTemplate restTemplate = new AsyncRestTemplate(); 

@Override
public ListenableFuture<String> getData(String userId) {
    final SettableFuture<String> responseFuture = SettableFuture.create();

    String hostname = "machineA";

    // execute the URL
    final org.springframework.util.concurrent.ListenableFuture<ResponseEntity<String>> orig = restTemplate
            .exchange(createURL(hostname), HttpMethod.GET, null, String.class);

    orig.addCallback(new ListenableFutureCallback<ResponseEntity<String>>() {
        @Override
        public void onSuccess(ResponseEntity<String> result) {
            responseFuture.set(result.getBody());
        }

        @Override
        public void onFailure(Throwable ex) {
            // how should I check whether my server is down/not responding. 
            // What is the concrete type of exception I should check here?
        }
    });

    return responseFuture;
}

Now my question is - Let's say if the server is down/not responding, then what kind of exception I will be getting in my onFailure method. I want to differentiate between 4xx/5xx error with server not responding/timing out. What is the concrete type I should check in my onFailure method to figure out whether my server was down or not responding.