do not discard status in fetch_refs_via_pack()

do not discard status in fetch_refs_via_pack()

The code calls fetch_pack() to get the list of refs it fetched, and
discards refs and always returns 0 to signal success.

But builtin-fetch-pack.c::fetch_pack() has error cases.  The function
returns NULL if error is detected (shallow-support side seems to choose
to die but I suspect that is easily fixable to error out as well).

Make fetch_refs_via_pack() propagate that error to the caller.

Acked-By: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>