@Test
public void getBalance_HttpResponseError_OperationFailedException() throws Exception {

    mockWebServer.enqueue(new MockResponse()
            .setResponseCode(500)
    );
    expectedEx.expect(OperationFailedException.class);
    expectedEx.expectCause(Matchers.<Throwable>instanceOf(HttpResponseException.class));
    getBalance(ACCOUNT_ID);
}