message: Add comment explaining window.windowErrors
diff: @@ -198,6 +198,7 @@ window.onerror = function (message, source, lineno, colno, optError={}) {
  // ignore
  }
  }
  + // window.windowErrors only exists if we're running in gui tests
  window.windowErrors && window.windowErrors.push([message, source, lineno, colno, optError]);
  };
  @@ -213,6 +214,7 @@ window.onunhandledrejection = function (e) {
  }
  }
  const {promise, reason} = event;
  + // window.windowErrors only exists if we're running in gui tests
  window.windowErrors && window.windowErrors.push([reason, promise]);
  };
  