pack-protocol: document "ERR" line
pack-protocol: document "ERR" line

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 <024f400cd820cb2ddbe54c7fecfd01e45f30bb96@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git