void propagateException() throws IOException {
        final Exception ex = exceptionRef.getAndSet(null);
        if (ex != null) {
            ClassicToAsyncSupport.rethrow(ex);
        }
    }
TODO: to be replaced by core functionality
httpclient5/src/main/java/org/apache/hc/client5/http/impl/compat/ClassicToAsyncResponseConsumer.java