Add method/option to check for JavaScript errors on page loads

(Works ONLY for Chrome (non-headless) and Chrome-based browsers such as Opera right now.)

Add method/option to check for JavaScript errors on page loads:

• New method:

self.assert_no_js_errors()

• New command line option (to automatically check for JavaScript errors after every page load):

--check_js

Ex:

pytest my_first_test.py --browser=chrome --check_js