Make the behaviour of the anonymous terminus clear when faced with transactional pre-settled messages

In the routing errors section of the anonymous terminus we discuss how the link to the terminus should be closed with an error if a message cannot be routed to the destination implied in the "to:" field, and the source does not allow for the rejected outcome. We should clarify this behaviour when messages are being sent within a transaction and have been sent pre-settled. In this case the behaviour defined for transactions (of essentially marking the transaction as rollback only) should take precedence.