Previously parsing a node caused the skin that was attached to it
to be parsed, which caused the skins node joints to be parsed,
which could cause the skin to be re-parsed leading to infinite or
at the very least exponential recursion.
The fix is to just get a reference to a temporarily uninitialized
skin as they were being parsed after the scene graph just to be safe
anyway. This way we avoid the recursion problem and all the references
will be valid in the end.
This can happen currently for example if NaNs are introduced in accessor bounds as rapidjson cannot write NaN/inf floats (see subsequent commit for fix there) and will halt writing to buffer at this point. Fix here ensures that whenever anything like this happens we throw an exception so this ends up as a registered export failure case, rather than silently exporting the incomplete JSON
