Fix unreasonable die in iostat_io

If `/proc/diskstats` is unreadable by some reasons, iostat_ios fallbacks to `/proc/partitions` and dies without give a change to read data from it with very weird reason. Better not to say users that their kernel isn't supported, but what feature is required to have such support.
Merge pull request #165 from kxepal/patch-1

Fix unreasonable die in iostat_io plugin for linux