Make 'too busy' an error in logs

lib/server.js

@@ -73,7 +73,7 @@ function requestHandler(req, res) {
       "Content-Type": "text/plain"
     });
     res.end("Server is over capacity :/");
-    logging.warn("Too busy:", req.id, 503, Date.now() - req.start + "ms", "(error)");
+    logging.error("Too busy:", req.id, 503, Date.now() - req.start + "ms", "(error)");
     return;
   }