Actually use pytest.raises in test.

tests/unit/test_serialize.py

@@ -31,4 +31,5 @@ def test_deserialize_new_json_cassette():
                    side_effect=UnicodeDecodeError('utf-8', b'unicode error in serialization',
                                                   0, 10, 'blew up'))
 def test_serialize_constructs_UnicodeDecodeError(mock_dumps):
-    jsonserializer.serialize({})
+    with pytest.raises(UnicodeDecodeError):
+        jsonserializer.serialize({})

vcr/serializers/jsonserializer.py

@@ -17,7 +17,7 @@ def serialize(cassette_dict):
             b"Error serializing cassette to JSON",
             original.start,
             original.end,
-            original.message +
+            original.args[-1] +
             ("Does this HTTP interaction contain binary data? "
              "If so, use a different serializer (like the yaml serializer) "
              "for this request?")