py : fix except (#5194)

ggml-ci

convert.py

@@ -336,7 +336,7 @@ class BpeVocab:
         self.bpe_tokenizer = json.loads(open(str(fname_tokenizer), encoding="utf-8").read())
         try:
             self.vocab = self.bpe_tokenizer["model"]["vocab"]
-        except:
+        except KeyError:
             self.vocab = self.bpe_tokenizer
         added_tokens: dict[str, int]
         if fname_added_tokens is not None: