public String errorCode() {
        return errorCode;
    }
Return the optional (but always non-empty if not null) error code as per
<a href="https://tools.ietf.org/html/rfc6749#section-5.2">RFC 6749: The OAuth
2.0 Authorization Framework</a>.

@return the optional (but always non-empty if not null) error code
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/OAuthBearerTokenCallback.java