The initial idea was to store exactly where the backups are stored.
But this only causes problems for migrations where the bucket might
change and clones where the prefix (box.id) changes.
Thus, it's best to leave the url creation to the caas side. (That
has to be done in another change)
the latest superchanged changed the meaning of 'error'. Previously,
error implied a network error. With the latest superagent, error means
a REST api error i.e 4xx, 5xx are flagged as errors.
error && !error.response means network error
