Preserve old values on error in setting API

Unfortunately WordPress does not provide the old value as an additional argument to your sanitize_callback callable. This is supported in general by the option validator hook that WordPress calls when saving the updated setting…

return apply_filters( "sanitize_option_{$option}", $value, $option, $original_value );

…but in practice it is not actually registered with any additional arguments when the filter is added during register_setting:

if ( ! empty( $args['sanitize_callback'] ) ) {
    add_filter( "sanitize_option_{$option_name}", $args['sanitize_callback'] );
}

Because add_filter doesn’t specify that the callable takes any additional arguments, apply_filter does not provide $option or $original_value to your callback when it is called (I consider this to be a bug, and have submitted a ticket to the developers). This means you cannot use the $original_value to fall back on when the provided setting is invalid. Instead, I suggest you just grab the current setting’s value within your callback:

if ( 'updated' === $type ) {
    return $newval;
} else {
    // use previous value
    return get_option( 'wpse3401_options' );
}