Fix routing of events related to replies to unroutable messages.

Errors seen that seem relevant:
```
UnroutableMessageError: ('Could not find next hop for event.', ...
```
and
```
'Error routing message for billing_dispatcher_ro'
```
and maybe
```
UnroutableMessageError: ('Outbound message for event has no associated user account:
```
I'm struggling to follow these two conditions. Can you add some explanatory comments?