add -i (built-in): send error messages to stderr

add -i (built-in): send error messages to stderr

The Perl version of that command already does that since a301973641f
(add -p: print errors in separate color, 2009-02-05). The built-in
version's development started by reimplementing the initial version from
5cde71d64af (git-add --interactive, 2006-12-10) for simplicity, though,
which still printed error messages to stdout.

Let's fix that by imitating the Perl version's behavior in the built-in
version of that command.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>