@Override
    public Feign.Builder invocationHandlerFactory(
        InvocationHandlerFactory invocationHandlerFactory) {
      throw new UnsupportedOperationException();
    }
Same as {@link #target(Class, String, T)}, except you can inspect a source exception before
creating a fallback object.
hystrix/src/main/java/feign/hystrix/HystrixFeign.java