When people return UnmarshalTypeError in UnmarshalJSON, we should append error's Field to FieldStack.

Fixes #68750

Change-Id: I0a5a9b259a1b569de1bebc815ec936c913e10469
GitHub-Last-Rev: 18796addc3fa0d367ba1a3f4bd268ca246890fe0
GitHub-Pull-Request: golang/go#68870
Reviewed-on: https://go-review.googlesource.com/c/go/+/605455


Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>