Validate parts

Per https://github.com/mapbox/mapbox-tile-copy/issues/56, and since zero-indexing can be a bit of a mindbend (for myself included), would be good to add a check ensuring that `--part` never equals or exceeds `--parts`.

Something like
```js
if (part >= parts) {
  return callback("woops");
}
```

cc @mapsam @millzpaugh 
  