self-signed leaf cert error message not clear

[JeyJeyGao]

notation-core-go/x509/cert_validations.go

Lines 43 to 54 in 0cb6e75

	// For self-signed signing certificate (not a CA)
	if len(certChain) == 1 {
	cert := certChain[0]
	if signingTime != nil && (signingTime.Before(cert.NotBefore) || signingTime.After(cert.NotAfter)) {
	return fmt.Errorf("certificate with subject %q was not valid at signing time of %s", cert.Subject, signingTime.UTC())
	}
	if err := cert.CheckSignature(cert.SignatureAlgorithm, cert.RawTBSCertificate, cert.Signature); err != nil {
	return err
	}
	return validateLeafCertificate(cert, expectedLeafEku)
	}

In line 50 or 53, if it return an error, it had better tell user that there is only one cert and the cert subject is xxxxx with error:xxxx
Also, it may be replaced by isSelfSigned() because it also need to make sure that the issuer and subject are the same.

[patrickzheng200]

The fix will be included in PR 120 as well.