Make error messages on typos and missing/incorrect data types more informative

[joelostblom]

When trying to reference a column from a dataframe and a typo is made, the error message suggests that there is an issue with the type specification rather than the column name itself:

import altair as alt
from vega_datasets import data

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

Old error

ValueError: Horsepowers encoding field is specified without a type; the type cannot be inferred because it does not match any column in the data.

I think that message is incorrect and unhelpful, and I often see learners struggle with understanding that they have made a typo. This PR suggests replacing the error message with this more informative alternative:

New error

ValueError: "Horsepowers" cannot be found.

• If you are trying to reference a column in the dataframe, there is likely a typo in the column name.
• If you are trying to reference a field from a transform, there is either a typo in the field name or the data type specification is missing (Altair can only infer data types from dataframe columns).

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Similarly, when making a typo in the encoding data type, the error message could also be made more informative:

alt.Chart(data.cars()).mark_point().encode(x='Horsepower:Qi')

Old error
ValueError: Horsepower:Qi encoding field is specified without a type; the type cannot be inferred because it does not match any column in the data.

This PR changes that to the following:

New error

ValueError: "Qi" is not one of the valid encoding data types: O, N, Q, T, G.
For more details, see https://altair-viz.github.io/altair-docs/user_guide/encodings/index.html#encoding-data-types. (If you are trying to use a column name that contains a colon, prefix it with a backslash, \:)

Note that this second change means that it is now required to escape colons in column names and although this is is a breaking change I think it makes it more consistent instead of having colon in column names sometimes work depending on whether the part after the colon happens to equal an Altair data type or not.

[joelostblom]

@mattijn How do you think we should proceed here? I think the improved error messages would be nice to have in the release candidate.

[joelostblom]

Merging in this too, yay better error messages everywhere!