crypto/x509: improve error when parsing bad ECDSA cert

when parsing an ECDSA certificate, improve the error message upon
failing to parse the curve as a named curve, rather than returning
the original ASN1 error.

Fixes golang#21502 (sort of, to the extent that it needs fixing)

src/crypto/x509/x509.go

@@ -1020,7 +1020,7 @@ func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{
 		namedCurveOID := new(asn1.ObjectIdentifier)
 		rest, err := asn1.Unmarshal(paramsData, namedCurveOID)
 		if err != nil {
-			return nil, err
+			return nil, errors.New("x509: failed to parse ECDSA parameters as namedCurve")
 		}
 		if len(rest) != 0 {
 			return nil, errors.New("x509: trailing data after ECDSA parameters")