Detecte login faild on facebook iOS sdk
<p>I'm using facebook SDK v4.12.0 in my iOS app. I want to detect if the user failed to login, how can I achieve this ? </p>

<p>This is my code </p>

<pre><code>FBSDKLoginManager *loginManager = [[FBSDKLoginManager alloc] init];
            [loginManager logOut];
            [loginManager setLoginBehavior:FBSDKLoginBehaviorWeb];

            [loginManager logInWithReadPermissions:@[@"email",@"user_birthday",@"user_friends"]
                                fromViewController:self
                                           handler:^(FBSDKLoginManagerLoginResult *result, NSError *error) {
......
}]; 
</code></pre>