CreateOrComment handler should accept emails without a subject or reply with a failure notice

Currently, the CreateOrComment handler will reject a message if it does not have a subject. While I understand the need to have a valid summary, in a helpdesk situation users often send quick support emails and forget to put in a subject line. Right now, there's no automated feedback that curbs that behavior. Furthermore, most users don't have very descriptive subjects for their support issues anyway.

I would rather have to have our support techs edit a bunch of tickets that come in with "(no subject)" text as the summary than to have those tickets go into limbo. We do have someone who gets the "EMAIL HANDLER: Error" emails (which is not well documented, btw), but that increases the time it takes to address an issue and does nothing to help us change users' behavior.

Ultimately, I'd rather have the handler accept the emails and use a standard (maybe even user-configurable) phrase such as "(no subject)" as the summary for those issues. Having users get a failure notice (as mentioned in issue JRA-2456) would be an acceptable, although not as desired, alternative. Having the ability to toggle between the two and possibly the current behavior would be ideal.