Fix: Add 'next' to error-handling middleware

[kfazinic]

Fixes TypeError: res.sendStatus is not a function when accessing non-existing routes.

Define error-handling middleware functions in the same way as other middleware functions, except with four arguments instead of three, specifically with the signature (err, req, res, next)):

https://expressjs.com/en/api.html ("Error-handling middleware")

[dpfernandes]

Approved