Bad respond msg

[uier]

https://github.com/Normal-OJ/Back-End/blob/672e6fcc9f9cdda7e15d765da31fb7baf11836e7/model/announcement.py#L40-L70

[Bogay]

wanted info:

• reason for failure.
• give the required parameters to tell users what happened. for example, consider follow response

  {
  "status": "err",
  "msg": "Announcement not found.",
  "data":
  {
    "courseName": "How to write oneline python",
    "errorType": "INVALID_COURSE_NAME"
  }
  }

  thses info are enough to client for deciding the UI.