How to send error http response in express/node js?

So in login page I am sending credentials from angular to express through get request.What I wanna do is that if found in database,send response and handle it in angular else if not found in db I want express to send error response and handle it angular error response function but my code isnt working.

Angular controller:

myapp.controller('therapist_login_controller', ['$scope', '$localStorage', '$http',
  function($scope, $localStorage, $http) {
    $scope.login = function() {
      console.log($scope.username + $scope.password);
      var data = {
        userid: $scope.username,
        password: $scope.password
      };
      console.log(data);
      $http.post('/api/therapist-login', data)
        .then(
          function(response) {
            // success callback
            console.log("posted successfully");
            $scope.message = "Login succesful";
          },
          function(response) {
            // failure callback,handle error here
            $scope.message = "Invalid username or password"
            console.log("error");
          }
        );
    }
  }
]);

APP.js:

  app.post('/api/therapist-login', therapist_controller.login);

Controller:

  module.exports.login = function(req, res) {

    var userid = req.body.userid;
    var password = req.body.password;
    console.log(userid + password);

    Credentials.findOne({
      'userid': [userid],
      'password': [password]
    }, function(err, user) {
      if (!user) {
        console.log("logged err");
        res.status(404); //Send error response here
        enter code here
      } else {
        console.log("login in");
      }
    });
  }