Add top level warning if page returned an error status code

[paulirish]

We have some checks in gather-runner to assert we have a network request for the document and it didn't completely fail:

lighthouse/lighthouse-core/gather/gather-runner.js

Lines 289 to 297 in 48a129f

	pageLoadError = GatherRunner.getPageLoadError(options.url, networkRecords);
	// If the driver was offline, a page load error is expected, so do not save it.
	if (!driver.online) pageLoadError = null;
	if (pageLoadError) {
	gathererResults.LighthouseRunWarnings.push('Lighthouse was unable to reliably load the ' +
	'page you requested. Make sure you are testing the correct URL and that the server is ' +
	'properly responding to all requests.');
	}

completely fail = Network.loadingFailed protocol event.

However 400+ and 500+ status codes will not be caught by this, but are probably an issue. We should add a top level warning in this case.

mainRecord.hasErrorStatusCode() just checks if the mainRecord.statusCode is >= 400. We might as well use this and add the mainRecord.statusCode into a toplevelwarning, so the user knows this pageload is not your typical one.

[paulirish]

dependency of #5881