Added URL check for HTTP Status on Screenshot Test launch page

In the past, we've noticed users running screenshot tests on pages that have issues being loaded in the requested browser. This then causes either failed screenshot results, or a screenshot of the browser showing the page could not be loaded for one reason or another.

Now when you launch a new screenshot test from our website, a call is made to check the HTTP status for the URL entered before the test is started. Once you click the "Take Screenshots" button, you will see a "checking url" animation display, and if the http status code is other than 200, a confirmation dialog is shown reporting the error details with the ability to cancel and make changes, or to continue the test with the URL as is. Good URLs will continue on to the result page as normal. Most URLs should be checked in milliseconds, but there is a 6 second time out on this function such that if for any reason the HTTP status check doesn't return in a timely manner, the screenshot test will continue without further interruption.

This saves users from the frustration of watching screenshots fail on all browsers requested due to common issues, like, the URL is misspelled, the username and password for basic authentication wasn't added to the URL yet is required, the SSL certificate has an issue, or the domain name cannot be resolved.

Hope this helps those of you who've had trouble with this in the past!