CiviMail: Sending a test email should not turn off exception handling and should handle errors intelligently

Currently, CiviMail test email goes through the same way as CiviMail mailing does, which means that CRM_Mailing_BAO_Job::deliverGroup() turns off error handling for the time of delivery. This is done so that any delivery-time issues (like hard bounces) do not stop the whole delivery process, but rather only mark the problematic recipient's email as 'on hold'.

We should extend this by adding logic that recognises test emails, and then does not turn the exception handling off, but rather tries to intelligently assess the error and notify the sender that the email was not send (and that there's an error) because, for example, it failed Mail_RFC822's validation. In other words, distinguish between recipient-specific failures (which should silently put the recipient's email on hold) and failures common to the whole mailing (which should be displayed to the user that attempts to send a test email).

Relevant forum thread: http://forum.civicrm.org/?topic=8400