Add message and more context to RequiredError

Created by: bodograumann

Changes the RequiredError exceptions class to retain the api class, method and field names as context. Most importantly the actual error message is now in the message field, where it belongs. Added an integration test to check it works. Verify with:

mvn integration-test -f samples/openapi3/client/petstore/typescript/tests/default/pom.xml

Fixes #10459 (closed)

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