core(gather-runner): include error status codes in pageLoadError

[brendankenny]

Adds a pageLoadError for 400/500 http status codes. Fixes #4082; depends on #6014 (see dcb468a for actual diff).

Example Report

Good:

• catches the error
• warns user at the top of the report that it happened and they should fix their URL
• issues runtimeError

Less good:

• redundant with the http-status-code SEO audit (new SEO audit: HTTP status code #3311). That audit successfully marks the page as 404ing (or whatever), but all the other audits around it are marked as Error! (since no gatherers in that pass return an artifact), so it's pretty much lost in the noise

• the SEO 403 smokehouse test is pretty much all broken, as another example of this

ref #5881

[brendankenny]

The redundancy might be fine, and we can just update the smoke test expectations, but it is another example of the tension between automatically failing gatherers (or not) in pageLoadError cases.

On the one hand, why is Lighthouse wasting time auditing stuff that was plainly in error (nothing like getting results for the "There was no service found for the uri requested." page)? On the other, why should Lighthouse care? Just warn me prominently but in the meantime give me the results for the page I asked for.

[paulirish]

HUGE. 👏
This also removes the need for the only remaining patch that LR applies on top of LH.

[brendankenny]

After discussion, we're going to maintain the change mostly as is.

I've split the SEO expectations into separate runs for HTTP 403 and other failures. That way we still get the end-to-end request failure test (including that it spawns error artifacts -> audit errors) while not losing all the other useful failure results to Error!.

[brendankenny]

appveyor failure is a transient chromestatus issue (no manifest found). Other error is bundlesize, which all the PRs are facing.