design/29934-error-values.md: promise vet check for As
design/29934-error-values.md: promise vet check for As

Change-Id: Ibd44b171c1366b0d312212d890a266c2a2faed81
Reviewed-on: https://go-review.googlesource.com/c/161997
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

golang/proposal,golang/proposal