[w3c/payment-request] Rename `PayerErrorFields` to be like `AddressErrors` and `BasicCardErrors` (#782)

`AddressErrors` and `BasicCardErrors` both end in "Errors" but `PayerErrorFields` doesn't and I also find that name a bit confusing (should it have been `PayerFieldErrors`?). It would be nice to have consistency and rename to `PayerErrors` (or `PayerFieldErrors` if `PayerErrors` is too confusing).

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