Proper way to handle errors in ExpressJS 3

I've been struggling for the past hours trying to find a decent tutorial on how to handle errors for a NodeJS + ExpressJS 3 api.

I've read a lot of questions/answers on SO and read the official ExpressJS guide and still cannot figure out the proper way to do such an important thing. Most of the answers I found were relevant in Express 1 or Express 2, but are not anymore and the ExpressJS Guide just describes something out of context, which makes it really hard to understand.

So here's my use case : I want to get informations about a city, using the GET /cities/:slug. If the slug doesn't exist or an error happened (with MongoDB or Mongoose for example), I want to return an error code.

Here's the relevant part of my current code :

if (err || !city) {
  res.json({code:500, city:[]});
}

It's a basic example and it's working "well", but I'm pretty sure that's not the right way to do it in NodeJS.

Also, this does not stop my NodeJS instance, but some unexpected errors might. From what I read, this is not a problem in production because apparently restarting the NodeJS instance is cleaner, and third-parties handle it pretty well (read about Cluster, Forever or even Monit).

Would love to have more insights on this problem I've been overlooking for too long.