F/location tag refactor

This branch contains a refactorization that extracts the formatting of the "location" name-value pairs associated with internal messages. This makes it trivial to attach configuration file references to error messages.

It introduces the function that makes this whole affair simpler and refactors call-sites where it already was used.

The series is based on my previous f/msg-warning-refactor branch, but is independent of that and could probably be rebased trivially.

Please merge!