Server-side validation for form

The default validation for the form I have works as expected. But when a user types in a valid email address and a password of three characters minimum, that doesn't mean the login credentials are valid.

So my question is:

How can I set the model email and password to invalid after server-side validation, so the input-fields get the class ng-invalid instead of ng-valid.

My current code

function IndexCtrl( $scope, $http )
{
  $scope.form = {};
  $scope.submitLogin = function ()
  {
    $http.post( '/api/auth/login', $scope.form ).success( function( data )
    {
      if ( !data.success )
      {
        $scope.form.errors = [ data ];

        // here I also want to mark the models 'email' and 'password' as invalid, so they both get the class 'ng-invalid'
      }
      else
      {
        $location.path( '/' );
      }
    });
  };
}