In commit
da404dc0f2 sync,copy: Fix --fast-list --create-empty-src-dirs and --exclude
The fix caused DirTree.AddDir to be called with the root directory.
This in turn caused a spurious directory entry in the DirTree which
caused tests with the -fast-list flag to fail with directory not found
errors.
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.