Improve unauthenticated error message

[fyockm]

The unauth'd error message only references adding an entry to .netrc. It should also mention BUF_TOKEN.

Failure: you are not authenticated. Create a new entry in your netrc, using a Buf API Key as the password.

Additionally, we should add a separate alert if the user of an invalid token in both BUF_TOKEN and .netrc. The former was already fixed; the latter was not. See https://github.com/bufbuild/buf/issues/1479#issuecomment-1642183614