Fix 0014630: Improved logging for emails

Prior to this, several e-mail related events were not reported at all,
making troubleshooting of issues difficult. The following cases are now
logged:

• sending of queued messages (email_send_all)
• errors thrown by PHPMailer
• reminder emails (email_bug_reminder) - fixes 0009368
• deletion of records from the email queue

In addition, admin/email_queue.php was modified to print a formatted
date instead of a numeric timestamp.