[calc_ratio] improve error message

[cdemulde]

When setting the only_checked argument to True, but when one (or both) of the data series haven't been checked yet, this currently give a KeyError, which is confusing to the user, as (s)he doesn't know it concerns the self.meta_valid DataFrame, not the original DataFrame. Improve this to warn the user that only_checked cannot be fulfilled.
To be decided: proceed with calculation for all data points and just give a warning, or throw an error and stop?