Don't assume all config read errors are missing files, print error

[jinnatar]

Fixes #18

Before: Config file: /etc/chadburn.conf not found
After: Cannot read config file: "warning:\ncan't store data at section \"global\", variable \"mail-to\"\n"

[jinnatar]

My original issue there of course is mail-to vs. email-to, this patch just makes it so that it's plausible to detect the issue instead of Chadburn claiming the file doesn't exist. :-)