A new class `NodeMessage` is used to wrap messages added to a node
during the mapping. This class will allow further features by giving
access to useful data related to the bound node.
BREAKING CHANGE: as of now every message is wrapped into a `NodeMessage`
it is therefore not possible to check whether the message is an instance
of `Throwable` — a new method `$message->isError()` is now to be used
for such cases.
