How can I know the error reason in promise's rejection handler?

How can I know the error reason in promise's rejection handler?

I wrote some code using find method like below. The adapter is RESTAdapter.
App.SessionManager = Em.Object.extend({
userIdChanged: function() {
App.User.find(this.get('userid')).then(
function(user) {
/* process something */
},
function(error) {
/* rejection handler */
/* I want to know what error returned by the server here.*/
/* but how? */
}
);
}.observes('userid'),
});
If my server returned some error(e.g. 401) the rejection handler called.
But it seems that the argument of handler doesn't have error information.
How can I know the error reason in rejection handler?