feat: Return ErrNotExist when a distro does not exist

[EduardGomezEscandell]

I read somewhere (I think some python style guide) that there are two patterns to access resources:

• Ask for permission: Check if the resource is available before using it.
• Ask for forgiveness: Try to use the resource and emit a meaningful error if it is unavailable.

The first is inherently racy as the availability of the resource may change after asking for permission. This lead our implementation to become an ask for permission, then ask for forgiveness sandwich which is needlesly verbose:

// Ask for permission
if ok, err := distro.IsRegistered(); err != nil {
    return err
} else if !ok {
    return ...
}

if err := distro.DoTheThing(); err != nil {
    // Ask for forgiveness
    return err
}

This PR makes usage a bit safer and less redundant:

err := distro.DoTheThing()
if errors.Is(err, wsl.ErrNotExist) {
    // Ask for forgiveness
    return ...
} else if err != nil {
    return err
}

Note that internally we do ask for permission in some places, essentially every Win32 API call. This is because the error reporting of these DLL calls is atrocious, and we cannot produce a meaningful error message from them.

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This is not for sport. The current implementation caused a test failure due to a resource availability race:

Failure: https://github.com/canonical/ubuntu-pro-for-wsl/actions/runs/8062559006/job/22022548242?pr=606#step:8:29

Code: https://github.com/canonical/ubuntu-pro-for-wsl/blob/f31559d796e8f2421acfbf4056e86e8f06854171/windows-agent/internal/distros/distro/properties.go#L34-L46

---

Part of UDENG-2385

[CarlosNihelton]

Makes sense to me.

[EduardGomezEscandell]

Force-pushed to rebase after #107. No new changes.