Add validation for the presence of adjustable

* Adjustments without an adjustable item do not make too much sense
* Enforcing an adjustable is always present will make it easier to find
  bugs in related code.
@dkubb I think its fine we validate the adjustable is actually loadable when validating. Also adjustments are only created (with providing an adjustable) or deleted. There is no situartion I know where the adjustable will get loaded on validation.