Job list failed for multiple jobs.
The url of the second job was the url of the first job with the second job identifier appended.
Because path() method of uri builder keeps appending.
Fixed by moving path() outside loop with a template variable.
Download sandbox even when job has an exception.
As output like stdout/stderr could contain details about exception.
Make clearer that status callback url is optional.
Octopus forgets about job after it has been cancelled.
See https://github.com/NLeSC/octopus/issues/73

Now `octopus.files().cancelJob(job)` method returns status so we use that.