Friendly error pages

I styled these pages to look just like the production site, with the main header and nav and all, but I personally didn't think it looked good. So I ended up sticking with the simpler version, and just used the Calagator font styles and colors for consistency.  

It's a bit playful, but isn't a little playfulness what everyone wants from an error page? :wink:

Fixes #45
Ideally, we want to move errors.css to app/assets/stylesheets, add it to the `config.assets.precompile` list in config/application.rb, and reference it as /assets/errors.css from the error pages.