Deduplicate error messages

[binste]

Fixes #2896 and this comment:

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

Further improvements to messages generated out of multiple errors can be tackled separately as part of #2873

[mattijn]

Thanks @binste!

After this PR, the following examples return as such:

import altair as alt
from vega_datasets import data

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

SchemaValidationError: 'invalid_value' is an invalid value for `sort`:

'invalid_value' is not of type 'array'

alt.Chart(...)

source = data.cars()
alt.Chart(source).mark_text(align="right").encode(
    alt.Text("Horsepower:N", bandPosition='4')
)

SchemaValidationError: '4' is an invalid value for `bandPosition`:

'4' is not of type 'number'
Additional properties are not allowed ('field' was unexpected)
Additional properties are not allowed ('bandPosition', 'field', 'type' were unexpected)

alt.Chart(...)

alt.Text("Horsepower:N", type=dict(text="Horsepower", align="right")).to_dict()

SchemaValidationError: '{'text': 'Horsepower', 'align': 'right'}' is an invalid value for `type`:

{'text': 'Horsepower', 'align': 'right'} is not one of ['quantitative', 'ordinal', 'temporal', 'nominal']
{'text': 'Horsepower', 'align': 'right'} is not of type 'string'

No duplication of error messages anymore. Will merge once tests pass.

One question, why did you had to add the $ sign?

[binste]

Great, thanks for double-checking! Regarding the $, the expected messages we use in the tests are treated as regex by pytest. $ matches the end of the line, else this test:

SchemaValidationError: 'invalid_value' is an invalid value for `sort`:

'invalid_value' is not of type 'array'

would also pass if the message looks like this:

SchemaValidationError: 'invalid_value' is an invalid value for `sort`:

'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'

I then also added it to the existing examples as an additional restriction just to be safe.