Api returns non-json response on 404 errors for some urls

[FilipChalupa]

It makes it hard to handle errors then. E.g. https://api-youdoit.eu.contember.cloud/future-feature responds with a string Not Found. I would suggest something like

{"errors":[{"message":"Not found","code":404}]}

to be consistent with other endpoints.

(I've discovered this this problem while trying to do https://api-youdoit.eu.contember.cloud/export with an old engine.)