cli: incorrect helper-id error is confusing

[philips]

I miskeyed a helper id while going through the README and the error made it sound like I didn't provide the flag when in fact I just had an incorrect ID. Also, it is weird the error message has underscores instead of dashes- some sort of CLI library thing?

$ divviup task create --name net-promoter-score --leader-aggregator-id c6a780d8-e0d3-4881-93dc-5b4af3712734 --helper-aggregator-id 96301951-c848-4a57-b4f5-32812e4db1be --collector-credential-id b24c97c9-1aff-4ff4-875c-cac4f6834b86 --vdaf histogram --categorical-buckets 0,1,2,3,4,5,6,7,8,9,10 --min-batch-size 100 --max-batch-size 200 --time-precision 60sec
Validation errors:
  - helper_aggregator_id:
    * required

$