Update JS examples (add catch)

[inancgumus]

We want to add catch blocks to our examples to introduce better examples for our users.

There is also another problem.

• In the following script, p.$ fails (I've changed the extension code to do so).
• However, we never catch that panic since it's in a try block—it swallows the panic.
• Then page.close panics since "p.$ panicked and shut down the browser already".
• So we receive only the page.close's panic even if p.$ says something like "I could not find the selector something."

import { check } from 'k6';
import { chromium } from 'k6/x/browser';

export const options = {
  thresholds: {
    checks: ["rate==1.0"]
  }
}

export default async function() {
  const browser = chromium.launch({
    headless: __ENV.XK6_HEADLESS ? true : false,
  });
  const context = browser.newContext();
  const page = context.newPage();

  try {
    await page.goto('https://test.k6.io/');

    check(page, {
      'Title with CSS selector':
        p => {
          console.log('checking page textcontext');
          return p.$('header h1.titlex').textContent() == 'test.k6.io'
        },
    });
  } finally {
    page.close();
    browser.close();
  }
}

Related: #590, #697