app.use(function* (next){
  try {
    yield* next;
  } catch (err) {
    if (401 == err.status) {
      this.status = 401;
      this.body = 'cant haz that';
    } else {
      throw err;
    }
  }
});

app.use(function *(next){
  try {
    yield next;
  } catch (err) {
    if (401 == err.status) {
      this.status = 401;
      this.set('WWW-Authenticate', 'Basic');
      this.body = 'cant haz that';
    } else {
      throw err;
    }
  }
});