Fixes for comparelibs.py

Created by: SchrodingersGat

(Also added extra verbose error messages for schlib.py)

The updates made in https://github.com/KiCad/kicad-library-utils/pull/52 have actually caused some errors as indicated here: https://github.com/KiCad/kicad-library-utils/issues/70

This PR fixes those errors and also reverts the script to the previous mode where it only runs on a single .lib file at a time.

Running the script on an entire batch of lib files should be handled by a separate wrapper script.