Improve unterminated block exception

[Sporarum]

Currently

raise FunctionDeclarationException(
    f"Missing return statement in function '{self.fn_node.name}'", self.fn_node

does not highlight the position of the missing return

This can be added by refactoring _is_terminated (the function called just before this check) to emit the test instead, and replace it's default return False by a raise which takes the current node a position (as opposed to currently the function node)