Switch scotty to production mode. Fixes #5.

From
  http://hackage.haskell.org/package/scotty-0.9.0/docs/Web-Scotty.html

  "Scotty is set up by default for development mode.
   For production servers, you will likely want to modify `settings`
   and the `defaultHandler`.
  "

We don't care about the "FD cache duration" mentioned in `settings`,
but we do want to add `defaultHandler` to not expose the contents
of IOExceptions to clients.