code: function errmsg(msg) {
    const obj = this._obj,
          errorMessages = getErrorMessages(obj);
    this.assert(
      _.includes(errorMessages, msg),
      'expected #{this} to have an error message: #{errmsg}',
      'expected #{this} to not an error message: #{errmsg}',
      msg,   // expected
      errorMessages   // actual
    );
  }
docs: Assert that target contains specified error message (unsuccessful validation).
  @param {String} msg - errmsg to match
  @example
  target.should.[not].have.errmsg(msg)