static HttpResponse badRequest(@Nullable String errorType) {
        if (errorType != null) {
            return HttpResponse.of(HttpStatus.BAD_REQUEST, MediaType.JSON_UTF_8,
                                   "{\"error\":\"%s\"}", errorType); //e.g."unsupported_token_type"
        } else {
            return HttpResponse.of(HttpStatus.BAD_REQUEST);
        }
    }
E.g.
   invalid_request
         <p>The request is missing a required parameter, includes an
         unsupported parameter or parameter value, repeats the same
         parameter, uses more than one method for including an access
         token, or is otherwise malformed. The resource server SHOULD
         respond with the HTTP 400 (Bad Request) status code.</p>
         Example:
         <pre>{@code
             HTTP/1.1 400 Bad Request
             Content-Type: application/json;charset=UTF-8
             {"error":"unsupported_token_type"}
         }</pre>
oauth2/src/main/java/com/linecorp/armeria/server/auth/oauth2/OAuth2AuthorizationErrorReporter.java