Harmonize error code for watchPosition()

[marcoscaceres]

Speaking with @saschanaz, they suggested we could harmonize all engines to just return 0 when whatPosition() would results in an error. This change would only affect Firefox, as Chrome and Safari already return 0, but the spec could be updated to reflect this.

[marcoscaceres]

Gave this some thought... I think we should just fix Gecko and leave the spec be.

[marcoscaceres]

Reopening the issue after some further discussion with @saschanaz and @annevk.

[marcoscaceres]

The argument presented is that web developers could come to rely on 0 being returned synchronously, so we might as well standardize that.

However, I'd argue that we should only ever return 0 going forward for any/all error and continue to push developers to check the GeolocationPositionError.code to figure out what actually went wrong.