(and `--json-error-messages`?) to provide more formalized machine readable serialization.

(weak) motivation: [https://github.com/datalad/datalad/issues/6431](datalad/issues/6431)

[[!meta author=yoh]]
[[!tag projects/datalad]]

> Added --json and --json-error-messages. Note that this did change the 
> --batch output in a way that could possibly break something that expected
> the old output to never change. I think it's acceptable to break that
> because there has never been a guarantee of unchanging output format
> except with --batch for most commands.
> 
> [[done]] --[[Joey]]
