Show errormessages on wrong username/password on custom loginform?

Why do you not use the default function wp_login_form() for the login form, example:

global $user_login;

if (is_user_logged_in()) {
    echo __( 'Hello', 'theme_text_domain' ) . $user_login . '<a href="' . wp_logout_url() . '" title="' . __( 'Logout', 'theme_text_domain' ) . '">' . __( 'Logout', FB_BASIS_TEXTDOMAIN ) . '</a>';
} else {
    wp_login_form();
}

the function and his params, more information on the codex

wp_login_form(array(
    'echo' => true,
    'redirect' => site_url($_SERVER['REQUEST_URI']), 
    'form_id' => 'loginform',
    'label_username' => __('Username'),
    'label_password' => __('Password'),
    'label_remember' => __('Remember Me'),
    'label_log_in' => __('Log In'),
    'id_username' => 'user_login',
    'id_password' => 'user_pass',
    'id_remember' => 'rememberme',
    'id_submit' => 'wp-submit',
    'remember' => true,
    'value_username' => ,
    'value_remember' => false
))