[jsonrpc] create_error(): allow null id
[jsonrpc] create_error(): allow null id

This allows you to call create_error() with a message argument that is
either None, or a notification. The JSON-RPC spec allows erros with a
null ID in case it is not clear where the error belongs to.

geertj/gruvi,geertj/gruvi,swegener/gruvi,swegener/gruvi