Instead of returning an error "error doing X", just return "doing X". The
fact that it's returned as an error object says enough about that it's
an error.
This also makes it easier to chain error messages, without seeing the
word "error" in every part of the chain.
Direct copy of the Flamenco Server Python code, for handling the change
of a job's status to trigger status changes on its tasks.
Not yet connected to the rest of the Manager logic.
The task status change → job status change code is a direct port of the
Flamenco Server v2 code written in Python.
There is no job status change → task status changes logic yet, and the
tests are also far from complete.
