Improve error message for unauthenticated error

[oliversun9]

This improves the error message when a BSR token is found in netrc but not valid, distinguishing it from the case where it's not found.

Fixes #2500

Message for no netrc entry and no BUF_TOKEN:

Failure: you are not authenticated for bufbuild.internal. Set the BUF_TOKEN environment variable or run "buf registry login bufbuild.internal", using a Buf API token as the password. For details, visit https://buf.build/docs/bsr/authentication

Invalid netrc entry:

Failure: your Buf API token for buf.build is invalid. Run "buf registry login" using a valid Buf API token. For details, visit https://buf.build/docs/bsr/authentication

Invalid BUF_TOKEN:

Failure: the BUF_TOKEN environment variable is set, but is not valid. Set BUF_TOKEN to a valid Buf API token, or unset it. For details, visit https://buf.build/docs/bsr/authentication