Throw an error if an invalid path is passed to -b and -c

[psivesely]

I accidentally just ran sudo paxrat -c /etc/paxrat.conf and was confused for a second why my changes hadn't taken place. Only after I looked through dmesg and /var/log/syslog did I notice I had typed the wrong path to my paxrat config.

[usefulz]

I think this was resolved with 843dd41