Add branded error pages

Currently the error pages for developers.redhat.com are missing/generic/unbranded. Notice that if you visit a missing page, (such as http://developers.redhat.com/blah) you see:

Not Found

The requested URL /products/foo was not found on this server.

A branded error page should be shown when a server error occurs, or if a page is not found.

Definition of done

• Visit http://developers.redhat.com/blah and check that a branded error page is shown.
• Simulating a server error is hard. I think we just have to trust that it works for that too.