Clean up NetconfMessage

NetconfMessage is currently rather not nice – it very much expects to have a document, but for error handling purposes it was broken by FailedNetconfMessage.

Repair that damage by eliminating FailedNetconfMessage and instead reporting a proper failure up the stack.

While we are in the area, also move NetconfNotification to netconf-api, so we have well-known messages co-located.

As a further evolution, rename NetconfNotification to NotificationMessage and NetconfHelloMessage to HelloMessage – cutting down on verbosity.