Expose machine-readable cause of error in callback result

Nobody has claimed this yet.

In case of an error in the bindings [we use SetErrorMessage](https://github.com/mapbox/node-or-tools/blob/289c9dbd4065717903e36fdc26356acabd70850a/src/tsp_worker.h#L30-L31) 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:

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

---

Implementation: in the async. workers override

```c++
void HandleErrorCallback()
```

and generate a Js Error object.

Start with src/tsp_worker.h and the SetErrorMessage usage linked in the issue, then inspect HandleErrorCallback in the asynchronous TSP and VRP workers. Confirm how NAN creates the JavaScript Error and make the callback expose both a machine-readable code and the existing message for each worker's error path.

Written by the indexing model from the issue text.