This is intended to help with adding more usage of anyhow in the
testsuite, which can help show context for errors.

This also includes some small improvements to the error messages to
provide more information.

This includes various minor refactorings to try to clean things up
and provide better error messages.

It isn't needed anymore, and I would prefer to not keep around unused code.
It can always be added back if ever needed again.

Use better high-level interfaces to achieve the same thing.

Using `with_json` is safer since it knows what JSON escaping is.

I don't trust that all these transformations won't have unintended
consequences on other platforms. It is nice to verify there aren't any
backslash shenanigans on other platforms.