Return 0 when watchPosition() errors

[marcoscaceres]

Closes #95 - @reillyeon, please see https://github.com/w3c/geolocation-api/issues/95#issuecomment-888988557 for rationale for change.

The following tasks have been completed:

• [x] Modified Web platform tests

Implementation commitment:

• [ ] WebKit - already returns 0.
• [ ] Chromium - already returns 0.
• [X] Gecko

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Preview | Diff

[marcoscaceres]

This change is included in test https://github.com/web-platform-tests/wpt/pull/29799

[marcoscaceres]

If I understand correctly you can check off implementation commitment from WebKit and Chromium as they already implement this behavior.

They implement the returning 0 part, yes - but not the "fully active" aspect of #78... so, to use a dated Simpsons reference, "partial credit" :)