Modify Java ApiException to have a more informative message.

Created by: brendandburns

Improves the implementation of Java ApiException for okhttp3 and top level in cases where there is a status code and a response body, but there is no message passed to the exception constructor.

See: https://github.com/kubernetes-client/java/issues/2066

cc @bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608

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