Enh: Improved message if a config file is missing

Older fpm versions allowed specifying configuration files with full path (e.g. the path of the file once installed, something like /etc/mypackage.conf ); current version requires a relative path and throws a rather cryptic "No such file or directory" error if the config file does not exist.

This patch provides a better message that will help upgrading users to understand what's happening.