Fix: sbd-md: return error if faied to list any devices

Based on the patch from Lidong Zhong <lidong.zhong@suse.com>

With this change, when using the lha stonith agent
"stonith:external/sbd", stonith resource will report errors rather than
be quiet if any sbd devices failed, so that users can realize. And it
won't trigger deregistration of the fencing device from
pacemaker-fenced, which makes sense -- We should try hard to use a
fencing device if there's any chance. If it really doesn't work,
it'll fail to fence then.

The issue was originally brought up from:
https://github.com/ClusterLabs/sbd/pull/40#issuecomment-353266808

Another possible solution of considering to return positive if a quorate
number of devices can be listed properly was suggested.

However so far all the "command" functions consistently return -1 when
failed, and then sbd consistently returns 1 as error for all the
commands.

We'd probably better not break the convention unless it's very necessary.