code: def _get_course_descriptor_path(self, courseid):
          
          if not id_checker(courseid):
              raise InvalidNameException("Course with invalid name: " + courseid)
          course_fs = self.get_course_fs(courseid)
          if course_fs.exists("course.yaml"):
              return courseid+"/course.yaml"
          if course_fs.exists("course.json"):
              return courseid+"/course.json"
          raise CourseNotFoundException()
docs: :param courseid: the course id of the course
          :raise InvalidNameException, CourseNotFoundException
          :return: the path to the descriptor of the course