pack-bitmap.c: using error() instead of silently returning -1

In "open_pack_bitmap_1()" and "open_midx_bitmap_1()", it's better to
return error() instead of "-1" when some unexpected error occurs like
"stat bitmap file failed", "bitmap header is invalid" or "checksum
mismatch", etc.

There are places where we do not replace, such as when the bitmap
does not exist (no bitmap in repository is allowed) or when another
bitmap has already been opened (in which case it should be a warning
rather than an error).

Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.c

@@ -326,6 +326,7 @@ static int open_midx_bitmap_1(struct bitmap_index *bitmap_git,
 	free(bitmap_name);
 
 	if (fstat(fd, &st)) {
+		error_errno(_("cannot fstat bitmap file"));
 		close(fd);
 		return -1;
 	}
@@ -350,8 +351,10 @@ static int open_midx_bitmap_1(struct bitmap_index *bitmap_git,
 	if (load_bitmap_header(bitmap_git) < 0)
 		goto cleanup;
 
-	if (!hasheq(get_midx_checksum(bitmap_git->midx), bitmap_git->checksum))
+	if (!hasheq(get_midx_checksum(bitmap_git->midx), bitmap_git->checksum)) {
+		error(_("checksum doesn't match in MIDX and bitmap"));
 		goto cleanup;
+	}
 
 	if (load_midx_revindex(bitmap_git->midx) < 0) {
 		warning(_("multi-pack bitmap is missing required reverse index"));
@@ -389,6 +392,7 @@ static int open_pack_bitmap_1(struct bitmap_index *bitmap_git, struct packed_git
 	free(bitmap_name);
 
 	if (fstat(fd, &st)) {
+		error_errno(_("cannot fstat bitmap file"));
 		close(fd);
 		return -1;
 	}