Raise an error if a non-existing field-name is specified together with a correct type

[joelostblom]

One thing I couldn't address in #2824 is when a non-existing column name is used with a correct data type. Currently that leads to an empty chart being created:

import altair as alt
from vega_datasets import data

alt.Chart(data.cars()).mark_point().encode(x='Horsepowers:Q')

It is not always easy to spot that there is a typo here, although there is some intuition in that if there is no data plotted then maybe there is an issue with the data column name. I don't know of an easy way to fix this because I think it suffers from all the same difficulties as mentioned in #2586 that it is complicated to determine which column names should be allowed in the spec. This might ideally be solved on the Vega-Lite side of things.

[mattijn]

This can't be solved on the Vega-Lite side either since by design it doesn't see the data.