audit suggestion: track failed HTTP requests

[alekseykulikov]

During page execution: ajax requests may fail, or images/js/css didn't exist and returned 404. That may be a solid recommendation to fix.
It's similar to the tracking of javascript errors #3109, 0 failed HTTP request is a valid expectation.

[patrickhulce]

HTTP failures result in console errors, so #2836 should actually address this as well.

[alekseykulikov]

@patrickhulce good point! I may try to extract changes from #2836 and provide new PR to add only this audit.

[patrickhulce]

fixed by #2836

[paulirish]

and provide new PR to add only this audit.

@alekseykulikov you could do that, though we may then have some interesting overlap between the two. It's a little awkward so we favor keeping the 2836 audit all-together. does that work for you?