cmd/golangorg: add xerrors to x.go
cmd/golangorg: add xerrors to x.go

Backporting the change from CL 159739 since updates to
x/tools/cmd/godoc are not automatically mirrored to cmd/golangorg

Updates golang/go#29206

Change-Id: Ic377255b321ab73cfb04513aa00a510e407a2bdf
Reviewed-on: https://go-review.googlesource.com/c/160137
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website