crypto/tls: improve error messages for invalid certificates and signatures

Also, fix the alert value sent when a signature by a client certificate
is invalid in TLS 1.0-1.2.

Fixes #35190

Change-Id: I2ae1d5593dfd5ee2b4d979664aec74aab4a8a704
Reviewed-on: https://go-review.googlesource.com/c/go/+/204157
Reviewed-by: Katie Hockman <katie@golang.org>

tls_test.go

@@ -1045,3 +1045,20 @@ func TestBuildNameToCertificate_doesntModifyCertificates(t *testing.T) {
 }
 
 func testingKey(s string) string { return strings.ReplaceAll(s, "TESTING KEY", "PRIVATE KEY") }
+
+// TestSupportedSignatureAlgorithms checks that all supportedSignatureAlgorithms
+// have valid type and hash information.
+func TestSupportedSignatureAlgorithms(t *testing.T) {
+	for _, sigAlg := range supportedSignatureAlgorithms {
+		sigType, hash, err := typeAndHashFromSignatureScheme(sigAlg)
+		if err != nil {
+			t.Errorf("%#04x: unexpected error: %v", sigAlg, err)
+		}
+		if sigType == 0 {
+			t.Errorf("%#04x: missing signature type", sigAlg)
+		}
+		if hash == 0 && sigAlg != Ed25519 {
+			t.Errorf("%#04x: missing hash", sigAlg)
+		}
+	}
+}