More informative debugging errors when credentials are not read properly

[kevinyu]

There seem to be several ways that things can go wrong and cause a cryptic error message of InvalidCredentialsError('Credentials file cannot be symbolic link') when reading the credentials file with g = GDriveCommands(PATH_TO_SETTINGS_YAML).

One example: if you rename the credential directory, you will need to modify settings.yaml > save_credentials_file path, and if you don't you will get the error that doesn't really describe the solution.

It would be better to catch these scenarios and give feedback to the user so they can fix the credential files or regenerate and not chase the error message that pops up that is irrelevant.