It was annoying me that our error payloads include explicit nulls, then over the weekend I spotted this fix in someone else's code. Works well, although I couldn't figure out how to write tests for it because we get the ApplicationError type back there, rather than the raw JSON string.

~/c/fxa-email-service (master) $ curl -d '{"subject":"bar","body":{"text":"baz"}}' -H 'Content-Type: application/json' http://localhost:8001/send
{"status":400,"error":"Bad Request","errno":null,"message":null,"data":null}

~/c/fxa-email-service (pb/strip-nulls-from-errors) $ curl -d '{"subject":"bar","body":{"text":"baz"}}' -H 'Content-Type: application/json' http://localhost:8001/send
{"status":400,"error":"Bad Request"}