[cpp-ue4] bug-fix issue 10205: response json parsing logging a error to unreal console when there is no actual parsing problem

Created by: leith-bartrich

This is my proposed fix to #10205 (closed) Fixes #10205 (closed)

In summary, rather than walk all vars, it walks required vars and non-required (optional) vars. And within the required vars, it differentiates between writeOnly and non-writeOnly vars. It allows optional or writeOnly vars to fail to parse (because they're absent) and not trigger a big red error log indication of a parsing failure. To be clear: The generated code should not yell in red about an error, when a server is actually following the schema. The balance here, is that it tries to parse the vars anyway, but doesn't yell unless the schema truly indicates their absence is critical.

There are many deficiencies here. e.g. A TryGetJsonValue call doesn't differentiate between a parse failure versus the absence of a value, in its return. Which forces a lowest common denominator solution here. e.g. It's not necessarily providing much runtime feedback where it could. However, I think both of those deficiencies are actually a design change, rather than a bug fix. They could be handled in a different issue and and discussed there.

Here, I'm only seeking to keep the generated code from logging errors for missing vars, that probably should be missing.

I moved the comments to keep the code looking nicer, since my original bug submission.

This change hasn't been tested against a large array of schemas. But it has been tested against the simple schema provided in the bug, against a dirt simple Django backend.

The code generated from the samples seems sane to me. But I'm not as familiar with that schema. Of note: the pet.yml model is really the only one that marks certain fields required for a response. However, this is why I think it should instead be logging more useful information to Verbose and VeryVerbose on a per field basis, rather than declare a ParsingFailure on the whole response object.

tagging: @Kahncode

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