Merge: More robustness in keep going

Merge: More robustness in keep going

A pass on the various modules of `nitc` to make them more robust when `keep_going` is activated.

The main changes are the introductions of the flags `is_broken` in the root classes ANode and MEntity. This flags simplify the clients code to check the validity of entities before working with them.

Pull-Request: #1746
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>