nvme-status: return negative status if !errno

nvme-status: return negative status if !errno

'errno' is set to errno value when system calls are returning with an
error (e.g., ioctl).  But, If a command fails without any system call,
then it needs to return the raw negative error value.

For example, if we don't provide any options for connect command:

	root@vm:~# nvme connect
	need a transport (-t) argument
	root@vm:~# echo $?
	0

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>