convert : fix TypeError on GPT-2 vocab.json

[likejazz]

  File "/llama.cpp/convert.py", line 1474, in <module>
    main()
  File "/llama.cpp/convert.py", line 1442, in main
    vocab, special_vocab = vocab_factory.load_vocab(args.vocab_type, model_parent_path)
  File "/llama.cpp/convert.py", line 1324, in load_vocab
    vocab = BpeVocab(
  File "/llama.cpp/convert.py", line 338, in __init__
    self.vocab = self.bpe_tokenizer["model"]["vocab"]
TypeError: 'int' object is not subscriptable

The error that occurred is TypeError, so I add one more exception type.
#5194

[likejazz]

@cebtenzzre @ggerganov When will it be merged?
Some of check failed with an error No space left on device, which is an error on GitHub's part, not mine.
https://github.com/ggerganov/llama.cpp/actions/runs/7775159375/job/21200806090?pr=5288