Set unknown error instead of null for res on end

lib/client.js

@@ -98,7 +98,7 @@ client.prototype.request = function(method, path, params, callback){
         });
 
         res.on("end", function(){
-            var error = null;
+            var error = new Error("An unknown error occurred");
             try{ data = json.parse(data); }catch(e){}
             if(data["errors"]){
                 error = data["errors"];