Add explicit assertion message when reform key error
Add explicit assertion message when reform key error

MalkIPP/openfisca-france-data,adrienpacifico/openfisca-france-data,openfisca/openfisca-france-data,openfisca/openfisca-france-data,adrienpacifico/openfisca-france-data,benjello/openfisca-france-data,benjello/openfisca-france-data,openfisca/openfisca-france-data,MalkIPP/openfisca-france-data