Raise error if atom asked to schedule with unknown intention

Instead of returning none from the schedule task/retry method
if a unknown atom intention is declared (this will cause later
failures) raise a execution failure when this happens to denote
that this is a problematic & exceptional case.

Change-Id: Id0e3211389e22459c5381f4a19d80d8d0baf1e25
Merge "Raise error if atom asked to schedule with unknown intention"