Overhaul internal messaging API

_context.py: Added helpers for the majority of MessageTypes,
start, failure and success have been avoided to avoid temptation to use
these throughout the code base. These should be called primarily from
timed_activity().

job.py, elementjob.py: Add helper functions to populate task_id kwarg. _fail() exists
purely to be overridden by ElementJob as it requires the scheduler kwarg
to be set to True.

Other: Switched to use _context.py message helpers throughout the code
base.