Themes preview: scan and warn of poorly-coded themes before activation

I think to help combat poorly-coded themes (i.e. missing required tags that destroy plugin functionality), it’d be awesome if the Theme preview would additionally scan and warn of a theme that doesn’t have the required tags, etc. — before activation.

There would be some overhead, but I think it’s worth it. As a plugin developer, the lack of wp_footer in footer.php has been a major persistent problem.