proceed past os.listdir errors in sorted_walk
ignore "System Volume Information" directory

This is a control directory on Windows whose permissions are always set very
restrictively:
http://blogs.msdn.com/b/oldnewthing/archive/2003/11/20/55764.aspx
fix crash when RG fields set to null

In general, we convert None values to type-based nulls in packed fields just
as we do for normal fields.