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
This will be done once (in the case of a recursive
call to handlechildren).