explicitly handle error when subject set is empty

[camallen]

linked to #3450

avoid trying to set NaN values in the DB (0.0 / 0.0), instead raise a custom error which we may ingore longer term via HB via something like

Honeybadger.configure do |config|
  config.exceptions.ignore << SubjectSetCompletenessWorker::EmptySubjectSet
end

Review checklist

• First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• Are all the changes covered by tests? Think about any possible edge cases that might be left untested.