crypto/x509: expose UnknownAuthorityError.Cert

This matches exposing CertificateInvalidError.Cert.
and (exposing but not the spelling of) HostnameError.Certificate.

Fixes #13519.

Change-Id: Ifae9a09e063d642c09de3cdee8a728ff06d3a5df
Reviewed-on: https://go-review.googlesource.com/32644
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/verify.go

@@ -102,7 +102,7 @@ func (h HostnameError) Error() string {
 
 // UnknownAuthorityError results when the certificate issuer is unknown
 type UnknownAuthorityError struct {
-	cert *Certificate
+	Cert *Certificate
 	// hintErr contains an error that may be helpful in determining why an
 	// authority wasn't found.
 	hintErr error

src/crypto/x509/verify_test.go

@@ -290,10 +290,15 @@ func expectUsageError(t *testing.T, i int, err error) (ok bool) {
 }
 
 func expectAuthorityUnknown(t *testing.T, i int, err error) (ok bool) {
-	if _, ok := err.(UnknownAuthorityError); !ok {
+	e, ok := err.(UnknownAuthorityError)
+	if !ok {
 		t.Errorf("#%d: error was not UnknownAuthorityError: %s", i, err)
 		return false
 	}
+	if e.Cert == nil {
+		t.Errorf("#%d: error was UnknownAuthorityError, but missing Cert: %s", i, err)
+		return false
+	}
 	return true
 }
 
@@ -1284,7 +1289,7 @@ func TestUnknownAuthorityError(t *testing.T) {
 			t.Errorf("#%d: Unable to parse certificate -> %s", i, err)
 		}
 		uae := &UnknownAuthorityError{
-			cert:     c,
+			Cert:     c,
 			hintErr:  fmt.Errorf("empty"),
 			hintCert: c,
 		}