Triggered before a partial is deleted.
The event handler can throw an exception to prevent deletion of a specific partial.
public function subscribeEvents()
{
Backend::$events->addEvent('cms:onDeletePartial', $this, 'partial_deletion_check');
}
public function partial_deletion_check($partial)
{
if ($partial->name == 'test')
throw new Phpr_ApplicationException("You cannot delete this partial!");
}