Make the API authentication error message more general.

[iamdharmesh]

Description of the Change

The PR updates the authentication error message to be more generic so that it can cover errors for both API key and access token authentication.

Closes #49

How to test the Change

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 and make sure it says "Authentication failed."

Changelog Entry

Changed - Make the API authentication error message more general.

Credits

Props @dkotter @iamdharmesh

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.