Stunner - Invalid expression in script tasks shown as "OK" when process saved

Invalid expression in script tasks shown as "OK" when process is saved.

1. Create a business object.
2. Create business process (start->script task->end).
3. Set a process variable with type of the created business object.
4. Try using the business object in expression but make a mistake. For example do not cast it to the proper type:

   ksession.getVariable("xyz")
   

5. Save the process.

Actual result 

Editor saves the process fine without showing any issues.
The logs tho show the compilation error caused by the bad expression in script task.

Expected result

I think the editor should be able to show this as well, since it is available it seems.

Acceptance criteria

TODO