ATM errors are output into stderr while json record lacks any hints on what went wrong

$> git annex add --json longfilenametogetlotsoferrorsandevenlonger4 
  longfilenametogetlotsoferrorsandevenlonger4: setFileMode: permission denied (Operation not permitted)
{"command":"add","success":false,"file":"longfilenametogetlotsoferrorsandevenlonger4"}

would be nice(r) to have

$> git annex add --json longfilenametogetlotsoferrorsandevenlonger4 
{"command":"add","success":false,"file":"longfilenametogetlotsoferrorsandevenlonger4", "msg": "setFileMode: permission denied (Operation not permitted)"}

or may be even an explicit "errormsg" or alike instead of just a generic "msg"