Make it impossible to action Tasks that don't have status 'waiting' or 'error'

[dotsdl]

It looks like it's currently possible to action Tasks that are any status. We don't want this to be possible, as only Tasks that can conceivably be run yet (status is 'waiting' or 'error'). It shouldn't impact downstream compute, but does create unnecessary ACTIONS relationships to a TaskHub that will likely never be cleared.

We should add a status check in the Cypher query for Tasks actioned in Neo4jStore.action_tasks. We will return None for any Task that was requested to be actioned but ultimately was not; this should be the resulting behavior with this change.