Plugin should exit with some other code than `0` if no disks are monitored

[asachs01]

When running the check with an exclude flag of -e 'ext4,xfs, there's effectively no disks being monitored. However, the plugin still exits with a 0, which can be misleading. The plugin should exit with some other exit status to indicate that there's effectively no output and that what's being monitored isn't actually monitored.