Used to report attempts to set an unsupported element When the attempt to set the element is made, the code does not not know the name of the task/type based on a mapping from the classname to the task/type. However one class may be used by a lot of task/types. This exception may be caught by code that does know the task/type and it will reset the message to the correct message. This will be done once (in the case of a recursive call to handlechildren).