onExceptionThrown(event) {
    const text = event.exceptionDetails.exception ?
          event.exceptionDetails.exception.description : event.exceptionDetails.text;
    if (!text) {
      return;
    }
    /** @type {LH.Artifacts.ConsoleMessage} */
    const consoleMessage = {
      eventType: 'exception',
      source: 'exception',
      level: 'error',
      text,
      stackTrace: event.exceptionDetails.stackTrace,
      timestamp: event.timestamp,
      url: event.exceptionDetails.url,
      scriptId: event.exceptionDetails.scriptId,
      lineNumber: event.exceptionDetails.lineNumber,
      columnNumber: event.exceptionDetails.columnNumber,
    };
    this._logEntries.push(consoleMessage);
  }
Handles exception thrown events.
@param {LH.Crdp.Runtime.ExceptionThrownEvent} event