fix(delete-env): proper warning on 404

[mtfurlan]

When we get a 404 deleting an environment there should be a better warning than "unexpected error encountered: HttpError: Not Found" Document in the readme that the user for the PAT must have admin on the repo, not just write or manage.

[mmkoles]

Thanks for the PR. It helped me to resolve why my env deletions are failing 👍

[sguha00]

@mtfurlan I've been strugging with getting delete-env to work. Does this work with fine-grained personal access tokens or just the classic tokens?

[mtfurlan]

Yeah, this repo works with PATs, but as I have in the readme change, the PAT needs repo scope permissions, and the owner of the PAT needs to be admin on the repo.