public static HttpResponse insufficientScopeErrorResponse() {
        return forbidden(INSUFFICIENT_SCOPE);
    }
Returns an {@link HttpResponse} with {@link HttpStatus#FORBIDDEN} result code and formatted
error response as below.
<pre>{@code
    HTTP/1.1 403 Forbidden
    Content-Type: application/json;charset=UTF-8
    {"error":"insufficient_scope"}
}</pre>

<p> This response indicates that 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>
oauth2/src/main/java/com/linecorp/armeria/server/auth/oauth2/OAuth2TokenScopeValidator.java