Fixes inappropriate use of stderr in monitor command handler.

While there, improve the messages some.

[Fixed Error **err -> Error *err local variable that broke the build.
--Stefan]

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1431691143-1015-10-git-send-email-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>