Describe the bug

This is a minor thing but noticed that if you connect via the new OAuth flow but then the access token becomes invalid for some reason, we show an error message on the settings page (different from the error admin notice).

This error message says the API key may be invalid, even if someone isn't using an API key at all (which will be the case for all new users once the OAuth feature ships).

Would be great to update that error message, either making it more generic (something like authentication failed instead of mentioning API key or access token) or modify the error message depending on if someone has an API key or access token set.

Steps to Reproduce

1. Authenticate via the new OAuth flow
2. In the database, modify the mailchimp_sf_access_token value to make it invalid
3. Refresh the settings page to see the error message

Screenshots, screen recording, code snippet

Environment information

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WordPress information

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