x/crypto/ssh: Wrap error in NewClientConn

[avorima]

What version of Go are you using (go version)?

$ go version
go version go1.15.10 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="auto"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/mv/.cache/go-build"
GOENV="/home/mv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/mv/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/mv/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/mv/.local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/mv/.local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build650852416=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/CdUXyBdUq2g

What did you expect to see?

Using type assertion or errors.As for things like *net.OpError on errors returned by NewClientConn works.

What did you see instead?

The returned error is a *errors.errorString due to the fact that it's returned using fmt.Errorf and %v instead of %w.

[networkimprov]

cc @FiloSottile

[avorima]

I see that simply changing the %v to %w won't work due to backwards compatibility issues, so I would propose adding a typed error in the ssh package that implements Unwrap to support error unwrapping for go versions > 1.13 and otherwise behaves like a normal errors.errorString.