fix: throw error if `getValuesFor` is called on a column that doesn't exist
fix: throw error if `getValuesFor` is called on a column that doesn't exist

doesn't fix any issues just yet, but makes a common problem more easily
identifiable

owid/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher