Throw on decode certificate errors

[tracyboehrer]

JwtTokenExtractor.validateToken

                    // Note that decodeCertificate will return null if the cert could not
                    // be decoded.  This would likely be the case if it were in an unexpected
                    // encoding.  Going to err on the side of ignoring this check.
                    // May want to reconsider this and throw on null cert.
                    X509Certificate cert = decodeCertificate(key.certificateChain.get(0));
                    if (cert != null && !isCertValid(cert)) {
                        throw new JWTVerificationException("Signing certificate is not valid");
                    }