Detail: handle non-existent arrays better.

Detail: handle non-existent arrays better.

If you call "mdadm --detail" with a device file for an array which
doesn't exist, such as by
  mknod /dev/md57 b 9 57
  mdadm --detail /dev/md57

you get an unhelpful message about and inactive RAID0, and return
status is '0'.  This is confusing.

So catch this possibility and print a more useful message, and
return a non-zero status.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@gmail.com>