p12n_extract.py - code refactor

Code has been heavily cleaned up (shortened lines to 80 characters, no string concatenations).

From a functional point of view: instead of generating a static .html file with errors in /p12n, I’m creating a separate ```errors.json``` file that can be used to create filterable views in PHP.

Also: I moved this script to its [own repository](https://github.com/flodolo/p12n-extract) with tests.