The dev server doesn't serve template provided error pages.

[waylan]

While working on #1039 I noticed that the dev server returns its own (unstyled) error page, rather than the error page provided by the template. It would be nice if that worked out-of-the-box.

As an aside, I realize that in various environments, the user will need to configure their production server and point it to the error page. However, users they might not even know a theme comes with an error page if they don't ever see it during development. The dev server should be serving a full, working site.