Define messaging-level error handling

[gnarea]

Or more specifically, define error handling in endpoint and gateway messaging channels. The service provider should always be fully responsible for the error handling in its channel.

There are a number of things that could go wrong at this level. For example:

• The sender might've used a channel session key that isn't recognised (it could've already expired).
• The recipient may not recognise the type of service message encapsulated in a parcel.

This is known as invalid message channel in async messaging.

See also #37.