Expose machine-readable cause of error in callback result

[daniel-j-h]

In case of an error in the bindings we use SetErrorMessage in both the TSP and VRP worker to let NAN generate an Js Error for us. What we should add is not only a message but also a parse-able code, think:

{code: 'Timeout', message: 'Unable to find a solution, solver timed out'}

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Implementation: in the async. workers override

void HandleErrorCallback()

and generate a Js Error object.