`/join` with a "via" `server_name` query parameter pointing to a server that is down gives CORS error

This issue has been migrated from [#15145](https://github.com/matrix-org/synapse/issues/15145).

---

### Description

*Spawning from https://github.com/vector-im/element-web/issues/24617*

Given two links to join the same room:

```
https://matrix.to/#/!gjhRIJfdhgjd:google.com
https://matrix.to/#/!gjhRIJfdhgjd:google.com?via=feneas.org
```

Without `via`, Element Desktop 1.11.23 shows a error dialog with `404` error code (it's okay, because server does not exist more, just links exist in chats). But with `via`, the error dialog is empty and there is a `502` CORS error (unexpected)

In the case where a Matrix server where room was being hosted does not exist more, in my opinion, we should return a 404 to the user. (I expect a 404 response for the link with `via`)

### Steps to reproduce

See above.

### Homeserver

`matrix.org`

### Synapse Version

`matrix.org`

### Installation Method

I don't know

### Database

`matrix.org`

### Workers

I don't know

### Platform

`matrix.org`

### Configuration

_No response_

### Relevant log output

```shell
above
```

### Anything else that would be useful to know?

_No response_

Start at the `/join` handling path and reproduce the two matrix.to links described in the issue, including the `via` server_name parameter pointing to a down server. Compare the responses for links with and without `via`; done means the failing case returns a 404 instead of an empty dialog with a 502 CORS error.

Written by the indexing model from the issue text.