daemon: give friendlier error messages to clients

daemon: give friendlier error messages to clients

When the git-daemon is asked about an inaccessible repository, it simply
hangs up the connection without saying anything further. This makes it
hard to distinguish between a repository we cannot access (e.g., due to
typo), and a service or network outage.

Instead, let's print an "ERR" line, which git clients understand since
v1.6.1 (2008-12-24).

Because there is a risk of leaking information about non-exported
repositories, by default all errors simply say "access denied or
repository not exported". Sites which don't have hidden repositories, or
don't care, can pass a flag to turn on more specific messages.

Signed-off-by: Jeff King <peff@peff.net>
Helped-by: Sitaram Chamarty <sitaramc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>