How to use mock $httpBackend to test error branch

I am following the AngularJS documentation from here

The problem is that the documentation describes only the "success/happy" branch of the code, and there is no example of how to test the "failure" branch.

What I want to do, is to set the precondition for triggering the $scope.status = 'ERROR!' code.

Here is a minimal example.

// controller
function MyController($scope, $http) {

  this.saveMessage = function(message) {
    $scope.status = 'Saving...';
    $http.post('/add-msg.py', message).success(function(response) {
      $scope.status = '';
    }).error(function() {
      $scope.status = 'ERROR!';
    });
  };
}

// testing controller
var $httpBackend;

beforeEach(inject(function($injector) {
  $httpBackend = $injector.get('$httpBackend');
}));

it('should send msg to server', function() {

  $httpBackend.expectPOST('/add-msg.py', 'message content').respond(500, '');

  var controller = scope.$new(MyController);
  $httpBackend.flush();
  controller.saveMessage('message content');
  $httpBackend.flush();

  // Here is the question: How to set $httpBackend.expectPOST to trigger
  // this condition.
  expect(scope.status).toBe('ERROR!');
});

});