### Please describe the problem.

see below.  expected: entries on failed copies with "success"=false, and not just error messages 


[[!format sh """

$> git annex copy --to=neurosmaug svgtune_0.2.0.orig.tar.gz nonexisting                         
git-annex: nonexisting not found                                       
copy svgtune_0.2.0.orig.tar.gz (checking neurosmaug...) ok
git-annex: copy: 1 failed

$> git annex copy --to=neurosmaug svgtune_0.2.0.orig.tar.gz nonexisting --json
git-annex: nonexisting not found
{"command":"copy","note":"checking neurosmaug...","success":true,"key":"SHA256E-s5121--6d8f7d10206a120a42bec2cd29bc2365d09889fdf070ac8c67d1cff0b1539f63.tar.gz","file":"svgtune_0.2.0.orig.tar.gz"}
git-annex: copy: 1 failed

$> git annex version
git-annex version: 6.20170220+gitg75a15e1ad-1~ndall+1

"""]]

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

[[!meta title="--json-error-messages could apply to file not found error"]]

> [[wontfix|done]] --[[Joey]]
