Document WebPageTest's results/test-run status codes

[stephendonner]

Self: put a pointer to, or include + point to, the canonical WebPageTest test-status / error / result codes, from here:

https://github.com/WPO-Foundation/webpagetest/blob/352833696d440d74f9f1008a6f1e6d807db4bdda/www/common_lib.inc#L2992-L3007

Error 7: "Invalid SSL Cert."
Error 99996: "Timed Out waiting for DOM element"
Error 99997: "Timed Out"
Error 99998: "Timed Out"
Error 88888: "Script Error"
Error 12999: "Navigation Error"
Error -2146697211: "Failed to Connect"

It's also, of course, worth vetting and trying to get more granular debug info about the failures, too.