Tolerate missing Podlove web player

Wordpress 3.5 clean install dies after podlove plugin activation if user forgot to add podlove web player.

> "PHP Fatal error:  require_once() [[a href='function.require']function.require[/a]]: Failed
> opening required 'player/podlove-web-player/podlove-web-player.php'
> (include_path='.:/usr/share/pear/php5.3') ..."

Leads to null output for / and /wp-admin
Using include_once instead keeps site and admin interface operable.