Fix the display of Add non-um user for TA/Librarian/Assistant

[melindakraft]

For TA/Librarian/Assistants only list students and observers roles for adding to the course, but currently, the list is populated to add all roles as Teacher, TA, Designer, Student, Observer.

@pushyamig states the settings file is as follows:
role.can.add.teacher={"roles": [{"role":"TeacherEnrollment"}, {"role":"DesignerEnrollment"}]}

Also, display with a correct error message when Backend checks if the non-um user added or not. Current behavior it just displays as if user added message but if in real non-um user added then section column update with right section name and if not added then section area remains blank. This is poor UI reporting.