uv3 throw new exception v2 binary delta deserialization

Version 3 models the v2 layout by scanning all reachable FSs; this drops (versus v2) unreachable FSs. Binary form deltas encode the changes relative to this model. Because the model for v3 doesn't match v2's (due to the omission of unreachable FSs), delta updates can't be handled when reading a v2-generated delta cas into a v3 instance.

Throw an exception for this case (using binary deserializing of Delta CASs generated using V2 into a V3 instance). Document this in the backwards compatibility section.