fix: retry()'s errorFields should be optional per WebIDL

[marcoscaceres]

closes #804

The following tasks have been completed:

• [x] Confirmed there are no ReSpec errors/warnings.
• [x] Modified Web platform tests (auto fixes on next IDL crawl)
• [x] Modified MDN Docs (nothing to fix, not covered at this level of detail)

Implementation commitment:

• [x] Safari
• [x] Chrome
• [x] Firefox - Our IDL binding layer caught this.
• [ ] Edge (public signal)

Optional, Impact on Payment Handler spec?

[marcoscaceres]

Thanks @romandev. I don't think it will have any impact on Chrome, but if you want to update the implementation, it's nice to keep things in sync!

@aestes, same.

[marcoscaceres]

Gecko details: https://bugzilla.mozilla.org/show_bug.cgi?id=1499992

[aestes]

WebKit details: https://bugs.webkit.org/show_bug.cgi?id=191212

[marcoscaceres]

Filed bug on Chrome. https://bugs.chromium.org/p/chromium/issues/detail?id=901703

[marcoscaceres]

Also fixed in Chrome! thanks @romandev for letting me know!