more useful error messages on some libconfig exceptions

Description

CJ requested more helpful error messages to help users fix their own config file issues, without needing to contact support.

What to focus on

• To test libconfig::SettingNotFoundException, edit a config file and remove a setting.
• To test libconfig::ParseException, delete a config file.
• I have not found ways to test the other two yet!

Reviewers

• @alexandros
• @amyblank
• @bz
• @tn

Resources

Issues

What issues does this fix?

• Fixes #21 (closed)