Raise error when many to many table is not found

[loganripplinger]

DMM = DataModelManagerSync(base)
        DMM.load_descriptor_into_sql_alchemy_model(skills_descriptor)
        DMM.load_descriptor_into_sql_alchemy_model(frameworks_descriptor)  # this will break if we call this first and/or only call this
        table_list = list(base.metadata.tables.keys())

        expect(table_list).to(equal(['skills', 'frameworks/skills', 'frameworks']))