"softer" json handling when downloading a mylist

[jaeseopark]

make nndownload throw FormatNotAvailableException when items is not present in mylist_json.

• before:

  KeyError: 'items'

• after:

  __main__.FormatNotAvailableException: Could not retrieve mylist info; response={"status":"ng","status_code":403,"status_message":"need_login"}