downgrade "packfile cannot be accessed" errors to warnings

These can happen if another process simultaneously prunes a
pack. But that is not usually an error condition, because a
properly-running prune should have repacked the object into
a new pack. So we will notice that the pack has disappeared
unexpectedly, print a message, try other packs (possibly
after re-scanning the list of packs), and find it in the new
pack.

Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -805,7 +805,7 @@ static int add_object_entry(const unsigned char *sha1, enum object_type type,
 		if (offset) {
 			if (!found_pack) {
 				if (!is_pack_valid(p)) {
-					error("packfile %s cannot be accessed", p->pack_name);
+					warning("packfile %s cannot be accessed", p->pack_name);
 					continue;
 				}
 				found_offset = offset;

sha1_file.c

@@ -2035,7 +2035,7 @@ static int find_pack_entry(const unsigned char *sha1, struct pack_entry *e)
 			 * was loaded!
 			 */
 			if (!is_pack_valid(p)) {
-				error("packfile %s cannot be accessed", p->pack_name);
+				warning("packfile %s cannot be accessed", p->pack_name);
 				goto next;
 			}
 			e->offset = offset;