code: public String throwableToString() {
          StringBuilder sb = new StringBuilder("Exception:  ");
          if (throwable != null)
              sb.append(throwable.toString());
          else
              sb.append("null");
          sb.append("\n");
          return sb.toString();
      }
docs: return Vertex throwable as String compatible with
  the way toString returns other information
  
  @returns String form of exception (or "none")