[zmarkdown] Fixes Sentry error handling

[Situphen]

Quoting Sentry's documentation:

The error handler must be before any other error middleware and after all controllers

I tried it on Zeste de Savoir's staging server using this custom route to generate an error:

app.get("/debug-sentry", function mainHandler(req, res) {
  throw new Error("My first Sentry error!");
});

Currently, the error is logged in .pm2/logs/ but not sent to Sentry. With these changes, it is logged and sent to Sentry.