Make directory iterators become end iterator on error

Make directory iterators become end iterator on error

	* src/filesystem/dir.cc (open_dir): Return same value for errors
	whether ignored or not.
	(_Dir::advance(error_code*, directory_options)): Return false on
	error.
	(directory_iterator(const path&, directory_options, error_code*)):
	Create end iterator on error (LWG 2723).
	(recursive_directory_iterator(const path&, directory_options,
	error_code*)): Likewise.
	* testsuite/experimental/filesystem/iterators/directory_iterator.cc:
	Update expected behaviour on error.
	* testsuite/experimental/filesystem/iterators/
	recursive_directory_iterator.cc: Likewise.

From-SVN: r241486