crypto/tls: better error messages.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/60580046

src/pkg/crypto/tls/common.go

@@ -9,6 +9,7 @@ import (
 	"crypto"
 	"crypto/rand"
 	"crypto/x509"
+	"fmt"
 	"io"
 	"math/big"
 	"strings"
@@ -540,3 +541,7 @@ func initDefaultCipherSuites() {
 		varDefaultCipherSuites[i] = suite.id
 	}
 }
+
+func unexpectedMessageError(wanted, got interface{}) error {
+	return fmt.Errorf("tls: received unexpected handshake message of type %T when waiting for %T", got, wanted)
+}