Improve kafka encode error

[rukai]

Drastically improves usefulness of error.
Previously:

shotover   00:08:06.689555Z ERROR connection{id=3 source="KafkaSource"}: shotover_proxy::server: failed to send or encode message: EncodeError

Now:

shotover   00:21:14.715015Z ERROR connection{id=2 source="KafkaSource"}: shotover_proxy::server: failed to send or encode message: Failed to encode kafka_protocol::messages::find_coordinator_response::FindCoordinatorResponse v4 body

Caused by:
    EncodeError

Matches the format of error given for a decode error.