ResourceTransformationDescriptionBuilder needs a rejectChildResource(PathElement pathElement) method

It has discardChildResource(PathElement pathElement) but in many, perhaps all cases using that is not correct.

Discarding is only valid if the legacy slave can produce a server without the discarded data and that server's running configuration will be consistent with newer version servers that see the data.

If that can't be done, or some conversion can't be made, a reject should happen to produce a more useful error message than what the slave will issue when it gets ops it can't understand.