Improve architecture of composing error messages for each validation error code

[westonruter]

Feature description

Add translatable error messages for each specific error code, removing the logic from \AMP_Validation_Error_Taxonomy::get_error_title_from_code() and into the respective sanitizer that originated the validation error. Improve structure of validation error to make it easier to construct messages. For example, harmonize node_attributes and element_attributes to just element_attributes.

So this is more of an architectural issue rather than providing just providing error message translations, which is already in progress. This involves adding validation error objects.

Related: #3664, #3780, #3817, #1420.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

This is going to naturally fall out from #1420.

[westonruter]

This makes sense to address as part of #3875 when extracting sanitization/validation logic to amp-toolbox-php.