Need handle non-exist user in app/auth.py

[42bchen]

function : def check_api_key()
If server can't find the user, need to response

Error {
  "status_code" : 404,
  "message" : "User doesn't exist"
}

[HuangJyunYu]

The service have response Authentication fail already.

[Bill Hsu]

To reproduce, place a non-exist user in the sender field.