use next() to report error instead of manual call

We stumbled over this while investigating senchalabs/connect#949: We have a middleware that tries to set a header, which throws an exception because of said issue in connect. While I don't fully understand why, the manual call to the errorHandler (instead of using it as a middleware) causes the server to crash in this case. By replacing the manual call with `next(err)` we can get both rid of this crash and only specify the `errorHandler` in [one place](https://github.com/airbnb/rendr/blob/master/server/server.js#L120) now.