ChangeStreams raise an error when the resume token is not found in the current change document however, the underlying cursor is left open.

We should close the cursor before raising an error here: https://github.com/mongodb/mongo-python-driver/blob/fdf4436201065e77a3d67c7c3d92be721bc7e20e/pymongo/change_stream.py#L116

Thanks emily.stolfo for pointing this out.