Allow usage with Sentry

I'm looking to set up an instance of the export server at my company and integrate it with our Sentry setup for error monitoring. In order to do that correctly, we need to be able to add the middleware after all routes/controllers are registered, but before any global error handling is set up that does not pass the error along to the next error handler.

Would you be open to adding a hook to add error handling middlewares [here](https://github.com/highcharts/node-export-server/blob/15421d34fd4577da19b7505756f76630dd05561e/lib/server/error.js#L45)?