AsserestFileAccess constructor
Construct a file access property for ftpPath and determine
is operated success or not.
ftpPath must be in absolute form or throws NonAbsolutePathException
if not obey.
Implementation
factory AsserestFileAccess(String ftpPath, bool success) {
  final pathUri = Uri.file(ftpPath, windows: false);
  if (!pathUri.hasAbsolutePath) {
    // Unaccept relative path
    throw NonAbsolutePathException._(pathUri);
  }
  return AsserestFileAccess._(
      UnmodifiableListView(pathUri.pathSegments), success);
}