midx: stop ignoring malformed oid fanout chunk

When we load the oid-fanout chunk, our callback checks that its size is reasonable and returns an error if not. However, the caller only checks our return value against CHUNK_NOT_FOUND, so we end up ignoring the error completely! Using a too-small fanout table means we end up accessing random memory for the fanout and segfault. We can fix this by checking for any non-zero return value, rather than just CHUNK_NOT_FOUND, and adjusting our error message to cover both cases. We could handle each error code individually, but there's not much point for such a rare case. The extra message produced in the callback makes it clear what is going on. The same pattern is used in the adjacent code. Those cases are actually OK for now because they do not use a custom callback, so the only error they can get is CHUNK_NOT_FOUND. But let's convert them, as this is an accident waiting to happen (especially as we convert some of them away from pair_chunk). The error messages are more verbose, but it should be rare for a user to see these anyway. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>