Use fetch() in a way that works for file URLs (#3071)

fetch returns a response code of 0 when it successfully loads a `file://` resource.

This means we can't just rely on `response.ok`.

Required for https://github.com/element-hq/element-call/issues/2994

src/config/Config.ts

@@ -13,6 +13,7 @@ import {
   type ConfigOptions,
   type ResolvedConfigOptions,
 } from "./ConfigOptions";
+import { isFailure } from "../utils/fetch";
 
 export class Config {
   private static internalInstance: Config | undefined;
@@ -74,14 +75,14 @@ async function downloadConfig(
   configJsonFilename: string,
 ): Promise<ConfigOptions> {
   const url = new URL(configJsonFilename, window.location.href);
-  const res = await fetch(url);
+  const response = await fetch(url);
 
-  if (!res.ok || res.status === 404 || res.status === 0) {
+  if (isFailure(response)) {
     // Lack of a config isn't an error, we should just use the defaults.
     // Also treat a blank config as no config, assuming the status code is 0, because we don't get 404s from file:
     // URIs so this is the only way we can not fail if the file doesn't exist when loading from a file:// URI.
     return DEFAULT_CONFIG;
   }
 
-  return res.json();
+  return response.json();
 }