[[!comment format=mdwn
 username="joey"
 subject="""comment 3"""
 date="2018-02-16T16:50:27Z"
 content="""
A problem with this idea is that error messages are relatively uncommon,
and if they're hidden away in an extra field of an existing json message,
it would be easy for the consumer to forget to handle that uncommon case.

Note that git-annex currently doesn't document the actual json structure it
uses, which is more or less ok because any given git-annex subcommand will
always output the same json structure (except some note fields that
only sometimes appear, whose data is targeted at humans).
It's self-documenting. Using json for errors breaks that pattern.
"""]]