Gracefully handle situation when a class or property is removed from an ontology

[szeke]

Currently this causes an exception and the model is left in a bad state.

One suggestion is that before executing a set semantic type or set link command, Karma checks whether the URIs for the properties are in the ontology. If they are not, then Karma should report an error to the user and skip executing the command. This can cascade an result on many commands being skipped, but that is OK.

In addition, it would be good if such bad commands got a marking in the command history (a red bug icon). Then the user can select them and delete them, or the user can add the missing classes and reload the model.