src/s_conf: More detailed error messages conforming to POSIX errno

When the configuration file is unreadable or not existing, charybdis will now report the POSIX error message from the failed call. This is a compromise between the behavior in f951460ae991e2e8defb9638f8ee508283705cbb and f6f049070e240d0ce637e9e3ac4fba4148b9725d.