Show python types instead of javascript types in error messages

[joelostblom]

Originally mentioned in #2842 (comment).

Currently data types in the error messages are javascript rather than python. For example an error message could look like this:

alt.Chart(data.barley()).mark_bar().encode(
    x=alt.X('variety'),
    y=alt.Y('sum(yield)', stack='null'),  # should be eg. stack=None
)

'null' is not one of ['zero', 'center', 'normalize']
'null' is not of type 'null'
'null' is not of type 'boolean'

A would probably be more useful if the ever measured message look like this:

'null' is not one of ['zero', 'center', 'normalize']
'null' is not of type NoneType
'null' is not of type Bool

Or even simplifying the NoneType further:

'null' is not one of ['zero', 'center', 'normalize']
'null' is not of type Bool
'null' is not None

There is a [dictionary in the code]( and #2842 (comment).) for conversion between javascript and python types that could be useful here, but we might need to create something new as well