Visual diff: check the `rootSelector` exists before making the EmbedAPI request

[humitos]

We perform visual diff over the rootSelector (default or defined by the user) at;

addons/src/docdiff.js

Lines 169 to 172 in 5b70166

	const oldBody = htmlDocument.documentElement.querySelector(
	this.rootSelector,
	);
	const newBody = document.querySelector(this.rootSelector);

If the rootSelector is not found in the current page, visual diff won't work. We can take advantage of this and do not perform the EmbedAPI request if we already know it's not gonna work 😄

We will need to check this at

addons/src/docdiff.js

Lines 142 to 147 in 5b70166

	promiseData = fetch(url).then((response) => {
	if (!response.ok) {
	throw new Error("Error downloading requested base URL.");
	}
	return response.json();
	});