sending a mail to a merge proposal with no subject gives a confusing error

[Robert Collins]

I sent a mail to an existing merge proposal on friday, and got back a confusing error.
--
Subject: Error Creating Merge Proposal
Body:
Your message did not contain a subject. Launchpad code reviews require all
emails to contain subject lines. Please re-send your email including the
subject line.
--

The confusion here is that this wasn't a new merge proposal, it was an existing one, so it should say something like 'could not add message to merge proposal xyz.' What might be better in fact is to treat a missing/empty subject line on a mail to an existing proposal as a having the same subject as autogenerated mails sent by launchpad-code about merge proposals.