Improve messages for multiple errors

[binste]

Based on feedback from @joelostblom in #2842 (comment):

On brief comment now, do you think it is confusing to see three error messages like in your example above? I don't know what would be a feasible solution technically here, but I am thinking that instead of

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

the message would be clearer as:

'asdf' needs to be one of ['zero', 'center', 'normalize'] or of type 'null' or boolean'

Also see subsequent comments in that thread. #2842 (comment) is also relevant.

[binste]

alt.Chart().encode(alt.Angle().sort("invalid_value")) gives:

'invalid_value' is not of type 'array'
'invalid_value' is not of type 'array'
'invalid_value' is not of type 'array'
'invalid_value' is not of type 'array'

Error messages should be deduplicated based on .message attribute. Furthermore, this should ideally show all literal values which are acceptable such as ["x", "y", ...].

Maybe one for the long list but we could investigate if it make sense to show the error messages of all leaves in this error tree that we're traversing to get the relevant error messages. Or should it be dependent on what kind of object the user provided as a value, e.g. if a string such as "invalid_value" is provided then it should show acceptable string values but if an unsupported type is passed then it should show which types are accepted?

[joelostblom]

I also saw some duplicated rounds of messages in #2896

[binste]

FYI, I'm working on this as part of #2915 and #2913. Feel free to assign to me