Switch from localized strings to error codes in TorConnect errors

As noted in !938 (merged), we try to translate some error messages in TorConnect.

However, it isn't great, because:

1. it's backend, localized strings don't belong there
2. it's backend, we need to pass stuff usable by the frontends in code (especially important for Android), it's the frontend's role to transform it in something for the users
3. for Android, this part of the code lives in GeckoView, which we currently don't translate