message: API: Add notification creation 404 handling
diff: import {Get, Post, Delete, Param, BodyParam, CurrentUser, Authorized,
  - UseBefore, JsonController, BadRequestError, ForbiddenError, NotFoundError} from 'routing-controllers';
  + UseBefore, JsonController, BadRequestError, ForbiddenError, NotFoundError, InternalServerError} from 'routing-controllers';
  import passportJwtMiddleware from '../security/passportJwtMiddleware';
  import {NotificationSettings, API_NOTIFICATION_TYPE_ALL_CHANGES} from '../models/NotificationSettings';
  import {Notification} from '../models/Notification';
  @@ -24,15 +24,15 @@ export class NotificationController {
  let unit: IUnitModel;
  switch (targetType) {
  case 'course':
  - course = await Course.findById(targetId);
  + course = await Course.findById(targetId).orFail(new NotFoundError());
  break;
  case 'lecture':
  - lecture = await Lecture.findById(targetId);
  - course = await Course.findOne({lectures: targetId});
  + lecture = await Lecture.findById(targetId).orFail(new NotFoundError());
  + course = await Course.findOne({lectures: targetId}).orFail(new InternalServerError('Course of lecture missing'));
  break;
  case 'unit':
  - unit = await Unit.findById(targetId);
  - course = await Course.findById(unit._course);
  + unit = await Unit.findById(targetId).orFail(new NotFoundError());
  + course = await Course.findById(unit._course).orFail(new InternalServerError('Course of unit missing'));
  break;
  default:
  throw new BadRequestError('Invalid targetType');
  