Feature: Add `script.onerror` handler.

[ryan-roemer]

• [x] Add onerror handler
• [x] Test errors down to IE8 (may be future work).
• [x] Figure out IE8 script.onerror story.
• [x] Update doc byte sizes.
• [x] Update doc usage stuff.

New API:

window._lload("http://example.com/foo.js", function (err) {
  if (err) { console.log("OH NOES", err); }
  // foo.js is loaded!
}/*, [optional context variable here] */);

/cc @PeoB

[exogen]

Just adding a note here that for old-IE we'll need a disclaimer, because onerror unfortunately just doesn't work. (Although, who knows, maybe everyone else just got that wrong too and we can find a way :wink:).

Having a timeout option could come in handy, especially in that old-IE case where it could be the stand-in for onerror. The problem with having a timeout is that you can't guarantee that the script won't still load after your timeout fires (there's no way to cancel the script loading across browsers AFAIK), so the code could still be executed after you've handled the timeout...

[ryan-roemer]

UPDATE: Error capture works so far for all but IE8 for a 404'ing script.

• FAIL: IE_8_Windows_2008_Desktop
• PASS: IE_9_Windows_2008_Desktop
• PASS: IE_10_Windows_2012_Desktop
• PASS: IE_11_Windows_2012_R2_Desktop