Serve 404 on missing files for serif dev.

This avoids exceptions that lead to 500 errors that result in a
stacktrace. For `serif new` sites, favicon.ico is missing but this
doesn't stop content from actually being up correctly. A potential
source of confusion.

See #22.
Merge pull request #22 from aprescott/dev/serve-404s

`serif dev` server should serve up 404s