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

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

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