block: don't unecessarily clobber bi_error for chained bios

Only overwrite the parents bi_error if it was zero. That way a successful
bio completion doesn't reset the error pointer.

Reported-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>