Make 404 page easier to understand

Fixes https://github.com/owncloud/core/issues/11133

Old:
![https://cloud.githubusercontent.com/assets/1846927/4305822/19462b02-3e7c-11e4-94f9-bc9d6a91bdf6.png](https://cloud.githubusercontent.com/assets/1846927/4305822/19462b02-3e7c-11e4-94f9-bc9d6a91bdf6.png)

New:
![screen shot 2014-09-17 at 22 50 20](https://cloud.githubusercontent.com/assets/878997/4311140/5501a24a-3eac-11e4-9c1f-7aa4c4bf9629.png)

@MTRichards @craigpg @rperezb