error_code per operation complex the interface too much

[lichray]

That does not feel good. But a 'traditional' global error code makes thread-safe coding hard.

My suggestions are:

1. Add an error_code member to directory_entry, *directory_iterator, and provide a member function like is_valid() for user to check the status;
2. Move the non-member operational functions into a context class, so that the error code can be managed in the same way.