The Nagios/Monitoring plugin check_win_net_usage just got a bugfix which catches connection errors to the target host.
In todays released version 20121019 the fix has been added and throws an UNKNOWN output when the target host is not reachable or in general when check_nt (used in the background) throws an error.
Thanks go to Hermit who caught the bug and delivered the solution to it.
Besides the plugin itselfm, the documentation also got an update. In the section "How to find the correct name of the network interface" I added the Windows command "typeperf.exe". The previously well working method (check_nrpe -c listCounterInstances -a "Network Interface") doesn't seem to work anymore with newer NSClient++ versions. So I spent some time researching for alternatives and typeperf does the job. The discussion of this issue can be followed in the comments on the article "How to monitor Windows network traffic with Nagios".