If Cause is called on a non-standard Context, call ctx.Err.
Fixes #62582
Change-Id: Iac4ed93203eb5529f8839eb479b6ee2ee5ff6cbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/527277 Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>