Add some helpful error messages

[scottyeager]

I ran into a couple errors while starting up mycelium. It would be nice to provide helpful messages to guide the user in these cases.

Error: NixError(EBUSY)

Happens when there's already a tun device in the slot mycelium is trying to use, for example when Yggdrasil or a VPN is running. Maybe this could be resolved by looking for a free index instead (tun0 is taken so try tun1, etc).

Error: NixError(EPERM)

Means we need to run with root permissions. I tried without at first as the readme suggests root isn't needed on Linux.

[mik-tf]

I confirm. I use a vpn and what worked for me was:

sudo mycelium --peers tcp://83.231.240.31:9651 quic://185.206.122.71:9651 --tun-name utun2