Issue 1807
InstructorFeedbackQuestionAddAction
- It is currently 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 for invalid questionNUmber
Remove catching and defining EntityDoesNotExistException since it is never thrown/not possible to be thrown by any method related.
Remove EntityDoesNotExistException since it is already caught by  function createFeedbackQuestion().
Added Assumption.assertNotEmpty/IsEmpty for String. Changed such assertions across TEAMMATES to use the functions.
undo some wrong changes
Added a modifyParamValue() function to make code in test/cases/ui/InstructorFeedbackQuestionAddActionTest.java clearer.
Added a TODO in FeedbackAbstractQuestionDetails to add a validate function.
InstructorFeedbackEditPageUiTest
+ Added getter for maxNumScaleBox in InstructorFeedbackEditPage
+ Added test for numscale validation to check javascript.
+ Added TODO for numscale, currently, user is able to submit when min/max/step is empty and trigger an error page.
+ Added some TODOs for mcq/msq
Merge branch 'master' of https://github.com/TEAMMATES/repo into Issue1810
Fixed getter, modified tests.