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

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>