app: avoid unicode errors when printing the "not a directory" error message

If the supplied repo contains unicode characters we would error out when
writing to stderr because the encodings don't match.

Let the core.stderr() function handle it.

Related-to: #305
Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>