Attempting to get a non-existent package from ghcr.io results in "authentication failure"

```
$ wkg get wasi:fake
No version specified; fetching version list...
Error: OCI error: Authentication failure: {"errors":[{"code":"DENIED","message":"requested access to the resource is denied"}]}

Caused by:
Authentication failure: {"errors":[{"code":"DENIED","message":"requested access to the resource is denied"}]}
```

Options:
- Find a way to distinguish between "not found" and (actual) "bad auth". This will require some research; I'm not confident that this is even possible with ghcr.io
- Add an option to the well-known `registry.json` indicating "this OCI registry doesn't use auth, so treat auth failure as 'not found'"
- Reword the error message to reflect the ambiguity, e.g. "Package not found or access denied"

Ref #37