title: Throwing Exception in a nested Flowable's onNext is causing an UndeliverableException
body: <p>I've code like this in a repository:</p>
  
  <pre><code>return Completable.fromAction {
      // Some code
      loginService.login(id)
          .subscribe(
              { response -&gt;
                  if(response.isNotSuccessful()) {
                      throw Exception()
                  }
                  // Some code
              },
              { e -&gt;
                  throw e
              }
      )
  }
  </code></pre>
  
  <p>I've code like this in a <code>ViewModel</code>:</p>
  
  <pre><code>fun onLoginAction(id) {
      repository.login(id)
          .subscribe(
              {
                  showSuccess()
              },
              {
                  showFailure()
              }
      )
  }
  </code></pre>
  
  <p>Basically, the <code>ViewModel</code> calls the login method in the repository which returns the <code>Completable</code>.</p>
  
  <p>This results in an <code>UndeliverableException</code> when the response is not successful. I want the <code>Completable</code>'s subscriber's <code>onError()</code> method to be called. How do I do this?</p>