Another inithook tweak

[JedMeister]

• Ensure that /etc/wireguard/wg0.conf exists - initialisation fails unless it exists; empty file is fine.
  • Note that wg0.conf should always exist when the inithook runs, but it's best to not expect things to be "as they should be"!
• Fix broken error message if wireguard-server-init.sh fails - cmd is string and should be passing proc.stderr

[JedMeister]

@OnGle

Following our offline discussion & your concerns, I have tested this more thoroughly and had a "proper" look at wireguard-server-init.sh.

I'm happy to give you more details, but the TL;DR is that beyond the error message fix, the other change I made addresses a highly unlikely - perhaps even impossible - edge case. It would only occur if the /etc/wireguard/wg0.conf had been removed after boot. If it did not exist at boot, the wg0 service would fail to start and the wg0 interface could not be up - so wg show interfaces would not list it. So that change - i.e. touching the conf file it if it does not exist - is of little to no value. OTOH touching the conf file would make no significant difference as wireguard-server-init.sh would overwrite the conf file anyway...

So the error message fix should be applied, but the other change probably should just be removed as it's cruft...