Introduce clear and meaningful validation errors

[manchenkoff]

The current library version uses prance as a validation tool, but there is no way to get appropriate errors content with details and causes.

[Andrew-Chen-Wang]

Is it possible with the introduction of pydantic that we could cut out prance and even cut out all dependencies?

[manchenkoff]

In theory, yes, the main idea of prance usage was to reuse basic parsing with references and all validation rules related to OpenAPI v2/v3 instead of defining it by myself. Unfortunately, I do not have that much time right now, but eventually, I want to refactor this approach and define all rules in Pydantic according to OpenAPI v3+ specifications only.