The docs says: 
```
The callback gets the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error and err.code will be the exit code of the child process, and err.signal will be set to the signal that terminated the process.
```
But there is no ```err```, only ```stderr```. And ```error``` can be ```null```. So I'm using ```error``` directly. Since it gives the info that we need for those ifs.