Generalize registry reloading & start introducing HTTPError

- Generalize registry reloading to most operations in storage.js
- Add an HTTPError class in rbUtil (renamed from util to avoid clashes with
  node's util module), and handle it in server.js
- Separate out response handling in server.js
- Disable a previously falsely passing test (which used baseURL instead of
  bucketURL). We don't fetch wikitext (yet), so don't test for it.

Fixes: https://phabricator.wikimedia.org/T76492
Starts fixing: https://phabricator.wikimedia.org/T76693