Obtuse error message appears if the server fails to respond to a delete device request

### Steps to reproduce


- Enable the "New session manager" in Labs
- Go to the "Sessions" tab in Settings
- Select a device or two and click "Sign out"
- Have a homeserver which will not answer this request in time, and thus the reverse proxy returns a 504 (you can simulate with a reverse proxy that always returns a `504` for `/_matrix/client/r0/delete_devices` requests.
- Receive the above error message, which is very confusing for a user.

### Outcome

#### What did you expect?

A `504` means the homeserver did not respond to the request quickly enough so the reverse proxy has timed the request out. The devices are eventually deleted server-side, but in a minute or two. A more helpful error message would be something along the lines of "The homeserver is taking a while to complete this request. Wait a few minutes and try again."

#### What happened instead?

I received this cryptic error with no recommended user action, leading to potential frustration.

### Operating system

NixOS

### Application version

Element version: 1.11.17, Olm version: 3.2.12

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

No