Start the hardening process and test for business logic invariants

[ryardley]

Business logic invariants should result in error events being sent locally on the event bus and systems should recover from invariants

Some to test for off the top of my head

• E3Request cannot be called multiple times for the same e3id
• Ensure Aggregators once closed cannot be open again
• Ensure new Aggregators cannot be created multiple times for the same e3id
• Strict event ordering