As seen in <https://github.com/datalad/datalad/pull/6510> datalad
is having to parse error messages to determine when nonexistant
filenames were passed to git-annex. It would be better if there was some
kind of API that it could use, since error messages can (and have) changed.

What would this API look like? It seems it could not be a special
exit status of git-annex, because exit status 101 is already allocated
for indicating when things like --size-limit and --time-limit have caused
it to skip processing certian files.

Perhaps something like `--json-error-unmatch` which would make it output
an additional line with a JSON record indicating that git ls-files
--error-unmatch had errored out.

Or `--json` could enable this additional JSON record output.
The advantage would be that datalad would not need to check the git-annex
version to see if the option is supported. It would need to output a
JSON record even when git-ls-files did not fail. Perhaps something like
this:

	{"git-ls-files exit status": 0}
	{"git-ls-files exit-status": 1}

This does risk breaking things that parse the existing JSON and fall
over on the new record, but I think git-annex should be free to add new
records and fields to its JSON output in general, and it has probably
at least added new fields before. --[[Joey]]
