Warehouse: Properly style the error pages

[alex]

This includes (but is probably not limited too):

• [ ] 500
• [ ] 404
• [ ] 410 (Maybe? Not sure if we'll ever use it.)
• [ ] 403

[dstufft]

One thing that I think would be kind of cute to do, is on the 404 page, embed a link to https://www.youtube.com/watch?v=cWDdd5KKhts as an easter-egg referencing both the "unofficial" name of PyPI and the fact that the skit itself is about a cheeseshop that's missing any cheese... essentially a 404 ;)

[dstufft]

Bonus points if we stick the real 404 content at the top, then lead into the embedded youtube with a "And now for something completely different".