code: public static FSError findNested(Throwable top)
      {
          for (Throwable t = top; t != null; t = t.getCause())
          {
              if (t instanceof FSError)
                  return (FSError) t;
          }
  
          return null;
      }
docs: Unwraps the Throwable cause chain looking for an FSError instance
  @param top the top-level Throwable to unwrap
  @return FSError if found any, null otherwise