Zookeeper browser incorrectly shows "code" and "error" as children when a node is not found

this is easiest to reproduce if you click one of the MESSAGES nodes under `//INSTANCES//MESSAGES`. Those are fairly ephemeral. the controller ZK api will return a 404, but the response has a "code" and "error" field. The ZK browser adds those as children. If you keep clicking them, it forever appends "code" and "error" children.

Start with the ZK browser code that turns controller ZK API responses into node children, using the 404 response from an ephemeral MESSAGES node as the reproduction case. Check the handling of the response's "code" and "error" fields; done means a missing node does not display those fields as children or append them on repeated clicks.

Written by the indexing model from the issue text.