Use MockWebserver to test error cases

TestHttpUnavailable.java

    @Test
    public void testDemoResponseError503() {
        reset(mainSceneMock);
        MainPresenterImpl presenter = new MainPresenterImpl(schedulersProvider, pokemonService);

        MockResponse response = new MockResponse();
        response.setResponseCode(HttpURLConnection.HTTP_UNAVAILABLE);

        getErrorMockWebServer().enqueue(response);

        presenter.onSceneAdded(mainSceneMock, null);

        testScheduler.triggerActions();

        verify(mainSceneMock, times(0)).setApiText(anyString());
        verify(mainSceneMock, times(1)).showErrorDialog("Fire on the Server");
    }