Incorperate more blocking before parsing as JavaScript

[annevk]

Before we try to parse as JavaScript (the final step, only useful for actual classic scripts as everything else will have caused this algorithm to return before then) there's more reasons we could block:

• https://fetch.spec.whatwg.org/#should-response-to-request-be-blocked-due-to-mime-type?
• https://fetch.spec.whatwg.org/#should-response-to-request-be-blocked-due-to-nosniff? (This is done, as step 13.)
• https://html.spec.whatwg.org/#fetch-a-classic-script checks if response's type status is an ok status