Revert error back into warning for byte fallback conversion. (#22607)

tests/utils/test_convert_slow_tokenizer.py

@@ -24,10 +24,12 @@ class ConvertSlowTokenizerTest(unittest.TestCase):
 
         original_tokenizer_with_bytefallback = FakeOriginalTokenizer(vocab_file=spm_model_file_with_bytefallback)
 
-        with self.assertRaises(RuntimeError) as cm:
+        with warnings.catch_warnings(record=True) as w:
             _ = SpmConverter(original_tokenizer_with_bytefallback)
+        self.assertEqual(len(w), 1)
+
         self.assertIn(
             "The sentencepiece tokenizer that you are converting to a fast tokenizer uses the byte fallback option"
             " which is not implemented in the fast tokenizers.",
-            str(cm.exception),
+            str(w[0].message),
         )