Settings API Message Error is global

The setting that is the first parameter is the not the option key. It is the key that you would’ve registered using register_setting

register_setting( 'settings_unique_key4172_91h, 'option_key', 'name_of_sanitiser_function' );

Then you can add settings errors like this, inside the callback

function name_of_sanitiser_function(){

    add_settings_error(
        'settings_unique_key4172_91h',
        esc_attr( 'settings_updated' ),
        $message,
        $type
    );

}

You might like a few tutorials. I agree personally that it is very confusing:

http://www.presscoders.com/2010/05/wordpress-settings-api-explained/

http://ottopress.com/2009/wordpress-settings-api-tutorial/