public class ResourceGraphException
A ResourceGraphException is thrown as a result of an operation that by itself
is legal (i.e. can be performed) but would result in an illegal state of the
resources in the OGEMA system.
For example, adding a reference to a valid target resource with the correct
type is a legal operation by itself, but may cause loops in which a resource
is its own sub-resource. Hence, the operation would not be performed and this
would be thrown.