Follow up on strict check for config file in environs/dotenv

[rptaylor]

Currently the chart invokes KAPEL without a config file, instead using environment variables: https://github.com/rptaylor/kapel/blob/master/chart/templates/cronjob.yaml#L73

Nevertheless it is possible, and for CLI testing could be useful, to use a config file:
https://github.com/rptaylor/kapel/blob/master/python/KAPEL.py#L266

Follow up on sloria/environs#294 to fail explicitly if a config file is requested but not found.

However I'm not sure if this handles the undesirable situation that a stray config file could get used accidentally: https://github.com/rptaylor/kapel/blob/master/python/KAPELConfig.py#L6

[rptaylor]

This could be improved by theskumar/python-dotenv#632 , but dotenv is upstream of environs.

This is interesting:
https://dev.to/proteusiq/trending-anti-pattern-loading-environments-j55

Maybe it would be better to migrate to https://github.com/pydantic/pydantic .

https://github.com/HBNetwork/python-decouple seems abandoned.