rrd2whisper.py: Handle bogus files
rrd2whisper.py: Handle bogus files

kerlandsson/whisper,acdha/whisper,alexandreboisvert/whisper,graphite-project/whisper,penpen/whisper,jjneely/whisper,graphite-server/whisper,obfuscurity/whisper,cbowman0/whisper,piotr1212/whisper,deniszh/whisper,akbooer/whisper