make unpack_object_header() non fatal

make unpack_object_header() non fatal

It is possible to have pack corruption in the object header.  Currently
unpack_object_header() simply die() on them instead of letting the caller
deal with that gracefully.

So let's have unpack_object_header() return an error instead, and find
a better name for unpack_object_header_gently() in that context.  All
callers of unpack_object_header() are ready for it.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>