unpack_callback(): use unpack_failed() consistently
unpack_callback(): use unpack_failed() consistently

When unpack_index_entry() failed, consistently call unpack_failed(),
instead of silently returning -1.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git