[BUG][JAVA] ApiException error code is private without getter

Created by: Xendar

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

The ApiException.mustache template provides a useful exception with code and name, but unfortunately the code is private (well, that's good actually) but there is no getter to access it, so we cannot really use it...

openapi-generator version

4.3.1

OpenAPI declaration file content or url

https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v3.0/petstore.yaml

Command line used for generation

java -jar openapi-generator-cli-4.3.1.jar generate -i https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/examples/v3.0/petstore.yaml -g jaxrs-resteasy -o gen

Steps to reproduce

Run the generator and open gen/java/org/openapitools/api/ApiException.java

Related issues/PRs

None found

Suggest a fix

Adding a basic getter should be enough