title: exception type in Observable.retrywhen
body: <p>i am new to rxJava, this is probably very stupid question but i am not able to figure out how to find the type of exception in retry when.
  i want to retry on a particular exception (eg Io Exception), but on others
  i want to pass the exception down the chain.</p>
  
  <pre><code>public Observable&lt;List&lt;String&gt;&gt; getData()
  {
      return apiConsumer.getData()
              .retryWhen(new Func1&lt;Observable&lt;? extends Throwable&gt;, Observable&lt;?&gt;&gt;()
              {
                  @Override
                  public Observable&lt;?&gt; call(Observable&lt;? extends Throwable&gt; observable)
                  {
                      // if(exception type == io exception)
                      return observable.delay(5, TimeUnit.SECONDS);
                      //else
                      //  dont retry, pass the exception to onError
                  }
              });
  }
  </code></pre>