Fixes HTTP 500 on file-not-found.

Intercepts Haml file-not-found template-rendering exceptions so that Errno::ENOENT doesn't cause the app to throw HTTP 500 when visiting, say, http://rubyconf.org.au/2015/speakerss or something.

(Fix does not appear in development due to https://github.com/sinatra/sinatra/issues/578; run `env RACK_ENV=production foreman start` to test as fake-production.)