public String errorUri() {
        return errorUri;
    }
Return the (potentially null) error URI as per
<a href="https://tools.ietf.org/html/rfc6749#section-5.2">RFC 6749: The OAuth
2.0 Authorization Framework</a>.

@return the (potentially null) error URI
clients/src/main/java/org/apache/kafka/common/security/oauthbearer/OAuthBearerTokenCallback.java