Treeviewer stalls if node_details API returns empty data

[hyanwong]

See #827. If the API is returning no data for leaves, the JS console is filled with:

TypeError in OZentry, when evaluating e.length

Not only does this fill up the console logs, but it completely stalls the tree viewer. We should handle this more gracefully.

[lentinj]

Not just "no data for leaves", any exception will cause node_details to return {}.

• The error is thrown away server-side
• The client doesn't check for missing entries in the response and falls over
• The client doesn't give up, it just keeps retrying

[hyanwong]

Perhaps we should return blank entries for each node ID instead, so that at least we don't keep on retrying?