we want to handle spawn errors here

```js
var proc = spawn(adbPath, args);

proc.on('error', function (err) {
  if (!alreadyReturned) {
    cb(err);
  }
});
```

with appropriate changes to work with the `alreadyReturned` var in this method.

also, don't we want to keep a handle on the process so we can end it cleanly at app end?