Aggregate some more thread columns properly.

- show thread as starred if at least one message in it is starred
- use arrival date of last message as thread arrival date
- use sum of message attachment counts in thread as thread attachment
  count
- show thread as answered/forwarded if all messages in thread were
  answered/forwarded
Merge pull request #240 from maniac103/thread-star

Show thread as starred if at least one message in it is starred.
Improve certificate failure notifications.

The commit that introduced those notifications also introduced a rather
... interesting design pattern: The CertificateValidationException
notified the user of its pure existance - it's no longer a 'message'
only, but defines policy. As this is more than unusual, replace this
pattern by the MessagingController treating
CertificateValidationException specially when accessing remote folders.

Also make clear which account failed when constructing the notification.