get_featured_media_thumbnail() is not compatible with featured galleries

The get_featured_media_thumbnail does not grab the first image from the gallery if no featured image is attached:

``` php
function get_featured_media_thumbnail( $post_id, $size = null ) {
    return get_the_post_thumbnail( $post_id, $size );
}
```

The requires that a featured image is always set for a featured gallery.
