bundle2: introduce a parttype attribute to BundleValueError We will use the Exception for more that just unknown part type.

bundle2: rename b2x:error:unknownpart to b2x:error:unsupportedcontent This is a backward compatibility breakage per se. But bundle2 was explicitly flagged as experimental, and this is one an error path anyway. So the worse possible outcome from this change is to still have a crash but with a different message.