Clarify failed <image> type loads

From @henke37 on CSSWG mailing list:

I think this will be an excellent opportunity to clarify what CSS-Images
[1] means with "If the UA cannot download, parse, or otherwise
successfully display the contents at the URL as an image".
In particular with the corner case of an image request having a response
with a 404 reply code, but an image type response body. HTML has some
non obvious ideas about this. [2] "Whether the image is fetched
successfully or not (e.g., whether the response status was an ok status
https://fetch.spec.whatwg.org/#ok-status) must be ignored when
determining the image’s type and whether it is a valid image." Unnatural
as it is, authors probably expect CSS and HTML images to be have
identically in this corner situation.

[1] https://www.w3.org/TR/css3-images/#invalid-image
[2] http://w3c.github.io/html/semantics-embedded-content.html#fully-decodable