Show the corresponding media stubs message

When playing media stubs, the "Please insert disc" dialog wasn't showing. This was because of a typo in the **IsPlaceHolder** property (the H wasn't capital) and **getPlayUrl** was returning the wrong URL.

This makes the **PLAY** method show the correct dialog, and **PLAYPlaylist** to show the "This item is not playable" dialog. I didn't do the same with that method because I don't know exactly what it does.

This came from this thread: http://mediabrowser.tv/community/index.php?/topic/10103-show-media-stubs-message/