drush --quiet cron generates watchdog notices
drush cron simply calls the Drupal cron function. Drupal is initiating the watchdog messages; Drush implements system_watchdog() to convert these messages to drush log entries (Drupal 6 and 7).  If you set the option --watchdog=0, then these messages will not be printed by Drush.

If there is a problem with cron, then $? should still be set to a nonzero value when drush cron exits, and the watchdog messages will still be available in the Drupal Watchdog log (visible via drush watchdog).

If you need a finer level of control over the error behavior of Drush cron, please submit a feature request in the issue queue.