Fix for Studio 404 and 500 page layouts

Fixes the layout of the 404 and 500 pages in Studio.
STUD-864

@cahrens  and @marcotuts can you review? If you want to see them in action, you can go here: 
http://studio.frrrances.m.sandbox.edx.org/404
http://studio.frrrances.m.sandbox.edx.org/500