code: public String getEventMessage() throws HelloSignException {
          JSONObject metadata = (JSONObject) get(EVENT_METADATA);
          String eventMessage = null;
          try {
              eventMessage = metadata.getString(EVENT_MESSAGE);
          } catch (JSONException ex) {
              throw new HelloSignException(ex);
          }
          return eventMessage;
      }
docs: Returns the message if any from the event
  
  @return String
  @throws HelloSignException thrown if there is a problem parsing the
  backing JSONObject.