Restore old documented functionality, add optional warning, document the warning

[rambo]

Summary

As per #2446 the new "raise by default" broke a ton of containerized apps where environment is used in the container but developers use .env files locally.

This will restore old documented functionality to a pattern that is just about everywhere but also adds an option to issue warning if the file is missing.

IDK why coverage on CI is being a silly bugger, my local coverage report says all modified code paths are covered

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

I prefer the approach on #2485.

Thanks for the PR.