Clean up InstructorFeedbackQuestionAddAction #1807

Updated pull request. Closed previous one at #1820.

Issue 1807
InstructorFeedbackQuestionAddAction
- It is (no longer) possible to add questions with empty question text by modifying the js validator script.(instructorFeedback.js:64)

+ Added assumption to FeedbackAbstractQuestionDetails to assume questionText not empty.
+ Added test for empty questionText, expect assumption faliure.
+ Added test for non-existent enumerations of giver/recipient/question type, expect assumption faliure.
+ Added test and assumption for invalid questionNumber
+ Removed redundant Exceptions.
+ Added Assumption.assertNotEmpty/IsEmpty for String.