Refactor OpenApiValidationException

[mxr576]

and create an OpenAPI spec validator service. This allows to decouple from justinrainbow/json-schema and ensure that the validate() method actually throws OpenApiValidationException validation errors not just set states in an object that we have to transform to exceptions.

• It should be the instance of the new module specific RuntimeException.
• It should be in the "Exceptions" namespace.