static HttpResponse forbidden(@Nullable String errorType) {
        if (errorType != null) {
            return HttpResponse.of(HttpStatus.FORBIDDEN, MediaType.JSON_UTF_8,
                                   "{\"error\":\"%s\"}", errorType); //e.g."insufficient_scope"
        } else {
            return HttpResponse.of(HttpStatus.FORBIDDEN);
        }
    }
E.g.
   insufficient_scope
         <p>The request requires higher privileges than provided by the
         access token. The resource server SHOULD respond with the HTTP
         403 (Forbidden) status code and MAY include the "scope"
         attribute with the scope necessary to access the protected
         resource.</p>
         Example:
         <pre>{@code
             HTTP/1.1 403 Forbidden
             Content-Type: application/json;charset=UTF-8
             {"error":"insufficient_scope"}
         }</pre>
oauth2/src/main/java/com/linecorp/armeria/server/auth/oauth2/OAuth2AuthorizationErrorReporter.java