It was already 'int' before the migration to msgpack:

https://lab.nexedi.com/nexedi/neoppod/-/blob/6332112cb/neo/lib/protocol.py#L653

It is 'int' after the migration to msgpack:

https://lab.nexedi.com/nexedi/neoppod/-/blob/9d0bf97a1/neo/lib/protocol.py#L842

(it's explicitly cast into an integer here)

Another verification that 'ErrorCode' is 'int' on the wire can be found
here:

https://lab.nexedi.com/nexedi/neoppod/-/blob/3ddb6663b/neo/tests/protocol#L76

In the Nexedi NEO protocol specification document v1 this is however not
specified (or I couldn't find it):

https://neo.nexedi.com/P-NEO-Protocol.Specification.2019