Unit testing - force error on reading response body

I've written http client wrapper in go and I need to test it thoroughly. I'm reading the response body with ioutil.ReadAll within the wrapper. I'm having a bit of trouble figuring out how I can force a read from the response body to fail with the help of httptest. I'm assuming I can set up a fake server as such:

handler := func(w http.ResponseWriter, r *http.Request) {
    w.WriteHeader(http.StatusOK)
}

ts := httptest.NewServer(http.HandlerFunc(handler))
defer ts.Close()

I'm assuming I need to modify the ResposeWriter. Now, is there any way for me to modify the responseWriter and thereby force the ioutil.ReadAll in the wrapper to fail?