code: public ConnectionManager onError(ErrorListener listener) {
          this.errorListener = listener;
          for (DataSiftConnection conn : connections) {
              conn.setErrorListener(listener);
          }
          return this;
      }
docs: /*
  Subscribes a callback to listen for exceptions that may occur during streaming.
  When exceptions occur it is unlikely we'll know which stream/subscription caused the exception
  so instead of notifying all stream subscribers of the same exception this provides a way to list
  the error  just once
  
  @param listener an error callback