Enable error handling in Java WebClient library, fixes #1243

Created by: lukstei

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Description of the PR

Enable error handling in Java WebClient library, fixes #1243 (closed). Also now the generated Exception class ApiException is used.

Custom error handling behaviour can be defined in an overwriting class e.g.:

public class MyApiClient extends ApiClient {
  @Override
  protected <T> Mono<T> handleUnsuccessfulResponse(ClientResponse response) {
    return super.<T>handleUnsuccessfulResponse(response)
      .onErrorMap(errorcode(404), x -> new MyNotFoundException("Not found"));
  }

  private static Predicate<Throwable> errorcode(int errorCode) {
    return x -> x instanceof ApiException && ((ApiException) x).getCode() == errorCode;
  }
}

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