Prompt user to check for expired token in auth error message

[nicksnyder]

A user in our public slack got confused when they suddenly started seeing this error message in the CLI:

Failure: you are not authenticated. Create a new entry in your netrc, using a Buf API Key as the password. For details, visit https://docs.buf.build/bsr/authentication

The problem was that their token was expired and buf registry logout fixed it for them.

This change improves the error message so that users know to check to see if their token has expired.