Detect and proxy inaccessible CDNs

[nametoolong]

EdgeCast has been blocked in China since 2014. Detection of unreachable CDNs and proxy fallback is pretty hard (see Lantern, which is often unusable due to issues in proxy selection, albeit they have been working on this for a long time), but it is a must on the way to being actually usable. Currently this prevents accessing any website that is hosted on an inaccessible CDN.