Raise a special exception for special messages.
    
    Today, we ignore koji messages with a None `task_id` in two ways.  First
    with a conditional, and then with a second check in the message
    `__init__` method.  This is a belt-and-suspenders approach.
    
    For other reasons in the UMB messaging plugin, we'd like to put this
    check in just one place and use a special exception in the initializer
    instead of in a conditional beforehand.