@Andy the script you suggested would work if I were having a python script (ex: errorhandling.py ) But I am not having any knowledge of python and so I would give up to adopt any alternative.
If only I could make sure non of the regex in the handlers referred to the particular folder /Codebox then I could assign
error_handlers:
- file: default_error.html as suggested here
https://developers.google.com/appeng...rror_Responses