connect.c: add a way for git-daemon to pass an error back to client

connect.c: add a way for git-daemon to pass an error back to client

The current behavior of git-daemon is to simply close the connection on
any error condition. This leaves the client without any information as
to the cause of the failed fetch/push/etc.

This patch allows get_remote_heads to accept a line prefixed with "ERR"
that it can display to the user in an informative fashion. Once clients
can understand this ERR line, git-daemon can be made to properly report
"repository not found", "permission denied", or other errors.

Example

S: ERR No matching repository.
C: fatal: remote error: No matching repository.

Signed-off-by: Tom Preston-Werner <tom@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>