[CSSOM][css-cacade] Style sheet loading should do ok status check

[annevk]

While working on https://github.com/annevk/orb I discovered that Chrome and Safari check if a style sheet has an ok status and otherwise ignore it. This does not appear to be defined in CSSOM.

(I don't think this poses a security problem as <script> exposes the same information and <object> does too.)