Redirect to play page for non-existent levels. Fixes issue #396

Accessing invalid level pages such as [http://localhost:3000/play/level/myfakelevel](http://localhost:3000/play/level/myfakelevel) will redirect the user to **levels** page with a warning message at the top. Refer to the attached screenshot.

![screen shot 2014-02-26 at 5 00 26 am](https://f.cloud.github.com/assets/1315101/2262991/122866d0-9e60-11e3-881a-0a237a2bea75.png).