btrfs-progs: more scrub cancel error handling

btrfs-progs: more scrub cancel error handling

If we request scrub cancel on an unmounted or
non-btrfs device, we still get a "scrub canceled"
success message:

# btrfs scrub cancel /dev/loop1
scrub cancelled
# blkid /dev/loop1
/dev/loop1: UUID="7f586941-1d5e-4ba7-9caa-b35934849957" TYPE="xfs"

Fix this so that if check_mounted_where returns 0
we don't report success.

While we're at it, use perror to report the reason for an open
failure, if we get one.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>