Stylesheet load events fire in many cases that shouldn't

[pshaughn]

Each of these tests fails on multiple cases because a load event is seen from a stylesheet that was bad in some way, including not being CSS, downgrading to http from https, wrong Content-Type, invalid Content-Type, and no ContentType with nosniff attribute,
WPT html/semantics/document-metadata/the-link-element/:

• link-load-error-events.html
• link-load-error-events.https.html
• link-style-error-01.html
• link-style-error-quirks.html
• link-style-limited-quirks.html

Servo does appear to avoid firing a load event when an import is a 404, which Chrome fails on, so we're getting some things right.