Add fallback handler for unknown response codes

lib/frenzy/http.ex

@@ -36,6 +36,9 @@ defmodule Frenzy.HTTP do
       {:ok, %HTTPoison.Response{status_code: 404}} ->
         {:error, "404 Not Found"}
 
+      {:ok, %HTTPoison.Response{status_code}} ->
+        {:error, "HTTP #{status_code}"}
+
       {:error, %HTTPoison.Error{reason: reason}} ->
         {:error, reason}
     end