Exit on config read error to avoid crash on EACCES

Exit on config read error to avoid crash on EACCES

Failure to read does not exit despite
`    /* Read the file. If there is an error, report it and exit. */`

EACCES (e.g. insufficient filesystem permissions) is enough to crash
on access through later `config_*()` such as those when either of
either of D-Bus, MPRIS or MQTT is used.

Seen `--with-mpris-interface` and
```
$ ls -l /etc/shairport-sync.conf
-rw-r-----  1 root  _shairport  28114 Jan 25 01:53 /etc/shairport-sync.conf
$ shairport-sync
Segmentation fault (core dumped)
```