Add styles for error page.
Add route handler for pages not specified in routes.json.

Visiting a route that's not specified in routes.json will result in an
error page.
Pass along error.css when rendering the error page.

Also, fix a bug where required assets were not pushed onto the embeds
array, but rather, replacing it.
Remove font-family from fw-thin rule.

This is not needed because the google fonts have specified the font-
family to use for font-weight:100.
Update error page with 404 message.