Difficulties fails / use websocket to be informed about a new block
It fails due to new block and the node answer a 504 error.
Using [Duniterpy](https://git.duniter.org/clients/python/silkaj/issues/7) may solve this issue as there is a `jsonschema` check.
For now, an exception could be triggered to avoid breaking the loop.
- [x] add `blockchain/difficulties` jsonschema in DuniterPy.
issue