go: crypto/tls: client auth failure alert codes can be improved

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

$ go version
go version go1.18 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/agandhi/Library/Caches/go-build"
GOENV="/Users/agandhi/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/agandhi/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/agandhi/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/agandhi/.gimme/versions/go1.18.darwin.amd64"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/agandhi/.gimme/versions/go1.18.darwin.amd64/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/8b/jm61yqyn0g126b4p23hrfz7c0000gp/T/go-build4159564691=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

1. Run a Go TLS server with ClientAuthType configuration higher than RequestClientCert. This could be an HTTPS server, gRPC, etc.
2. Connect to it using curl, openssl s_client, etc and provide an invalid client certificate.

What did you expect to see?

I’d like to see one of the more meaningful TLS alerts that RFC 5246 (TLSv1.2) or RFC 8446 (TLSv1.3) define. Namely:

• certificate_required when the server is configured with tls.RequireAnyClientCert or tls.RequireAndVerifyClientCert, and the client doesn’t provide any client cert
• certificate_expired when the client provides a client cert but it’s expired (or not yet valid)
• unknown_ca when the client provides a client cert but it’s not signed by an authority that the Go TLS server is configured to require.

What did you see instead?

TLS alert bad_certificate is always returned, which is often confusing to end users because it doesn’t surface enough information to quickly find out if/how they can fix their client certificate.