try/catch in HandleOKCallback

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We should add try/catch around code in the HandleOKCallback like https://github.com/mapbox/node-cpp-skel/blob/c3d13e76cfb1178c92bf8beca822da4aba8c28f7/src/object_async/hello_async.cpp#L179-L186. Otherwise down stream developers are likely to:

- add new code to the HandleOKCallback
- in many instances that code might throw
- a throw will not be caught automatically and will instead crash the process with an `abort`

An example is https://github.com/mapbox/vtquery/issues/69 /cc @mapsam

We must protect against crashes like this, so I think its worth adding try/catch around the code in node-cpp-skel. Then users would be forced (due to the lack of coverage of the `catch` to think hard about whether an exception is possible.

Start with the referenced src/object_async/hello_async.cpp example, especially lines 179-186, and locate HandleOKCallback in node-cpp-skel. Add the requested exception handling around its callback code, then verify that downstream exceptions are caught rather than aborting the process.

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