Handle errors in response bodies

[annevk]

And also clean up the way we manage XMLHttpRequest's response, which allows for simplifying "handle response end-of-body" and "handle errors".

Tests: https://github.com/web-platform-tests/wpt/pull/27778.

Fixes #314.

• [x] At least two implementers are interested (and none opposed):
  • Chrome (presumably)
  • Safari (presumably)
• [x] Tests are written and can be reviewed and commented upon at:
  • See above.
• [x] Implementation bugs are filed:
  • Chrome: N/A
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1695216
  • Safari: N/A

(See WHATWG Working Mode: Changes for more details.)

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[annevk]

(@domenic I think this ends up fixing something you mentioned in passing as well about the response parameter. The whole thing is still a bit clunky, but this makes it a lot better.)

[annevk]

@yutakahirano is this on your radar?

[yutakahirano]

Oh sorry, I'm looking...