code: @Nonnull
    public static FileIOError deleteDirIfExisting (@Nonnull final File aDir)
    {
      final FileIOError aError = deleteDir (aDir);
      if (aError.getErrorCode ().equals (EFileIOErrorCode.SOURCE_DOES_NOT_EXIST))
        return EFileIOErrorCode.NO_ERROR.getAsIOError (EFileIOOperation.DELETE_DIR, aDir);
      return aError;
    }
docs: Delete an existing directory if it is existing. The directory needs to be
  empty before it can be deleted.
  
  @param aDir
  The directory to be deleted. May not be <code>null</code>.
  @return A non-<code>null</code> error code.
  @see #deleteDir(File)