Add Error Message when wireguard-go not found. replacement for #232

[noyez]

Previously when trying to add a peer, i for the following output, which
was not helpful since i wasn't sure what file could not be found.

bkn@mbp16 bkn % sudo /Users/bkn/.cargo/bin/innernet  --verbose  install mbp16.toml
✔ Interface name · chester
[*] bringing up interface chester.
[E] failed to start the interface: chester - No such file or directory (os error 2).
[*] bringing down the interface.
[!] failed to bring down interface: chester - WireGuard name file can't be read.
[E] Failed to redeem invite. Now's a good time to make sure the server is started and accessible!

[E] chester - No such file or directory (os error 2)

After changes:

[1] bkn@mbp16> sudo target/debug/innernet --verbose  install /Users/brad/mbp16.conf             /Users/brad/source/rust/innernet
✔ Interface name · chester2
[*] bringing up interface chester2.
[E] failed to start the interface: chester2 - Cannot find "wireguard-go". Specify a custom path with WG_USERSPACE_IMPLEMENTATION..
[*] bringing down the interface.
[!] failed to bring down interface: chester2 - WireGuard name file can't be read.
[E] Failed to redeem invite. Now's a good time to make sure the server is started and accessible!

[E] chester2 - Cannot find "wireguard-go". Specify a custom path with WG_USERSPACE_IMPLEMENTATION.

ref new PR to replace #232

[noyez]

On a semi-related topic you might also consider adding functions to output the env-vars of WG_USERSPACE_IMPLEMENTATION, like so:

fn wg_userspace_implementation_env_var() -> &'static str { "WG_USERSPACE_IMPLEMENTATION" }
fn wg_quick_userspace_implementation_env_var() -> &'static str { "WG_QUICK_USERSPACE_IMPLEMENTATION" }

this will eliminate the same hardcoded strings in multiple locations throughout the code base. As someone who is prone to typos, this i had misspelled one of those env vars and it stumped my testing for a bit, so i thought why not just put those in a function (or a const or a const fn) or something.

[skywhale]

LGTM

I'm sorry that the PR is left unreviewed for a long time 🙇

[bschwind]

Also sorry for an incredibly late review on this, but it looks good! I just have on minor point to address.

[bschwind]

We can probably pull get_userspace_implementation() into a local var called userspace_implementation and then use it in Command::new() above, as well as here:

Suggested change

	"Cannot find \"wireguard-go\". Specify a custom path with WG_USERSPACE_IMPLEMENTATION.",
	"Cannot find {userspace_implementation:?}. Specify a custom path with WG_USERSPACE_IMPLEMENTATION.",