Since a807328 (connect.c: add a way for git-daemon to pass an error
back to client), git client recognizes "ERR" line and prints a
friendly message to user if an error happens at server side.
Document this.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>