Threading woes in relation to unsuccessfully sent messages

### Steps to reproduce

1. Send a message (M1) to a thread, which fails to be delivered (HS outage)
2. (bug 1) Get warning about some messages not being sent, but there's no indication of *which* due to the thread UI.
3. Write a new message (M2) to the thread *before* clicking to delete all unsent messages.
4. Click to delete all unsent messages.
5. (bug 2) Both messages (M1 and M2) disappear in the thread view. However, M2 is still shown in the thread "pill" in the main timeline, even though it was never sent:


^ This message is not visible in the thread itself and was not received by other users.

### Outcome

#### What did you expect?

- Bug 1: I expected to be shown an indicator about which message(s) were unsent.
- Bug 2: I expected M2 to go through even though M1 was unsent. I expected only M1 to disappear when I clicked delete. And even if both should disappear, then M2 shouldn't be shown in the thread pill in the main timeline either.

#### What happened instead?

- Bug 1: See step 2.
- Bug 2: See step 5.

### Operating system

Arch Linux

### Application version

Element version: 1.10.12 Olm version: 3.2.8

### How did you install the app?

Arch Linux repository

### Homeserver

termina.org.uk, Synapse 1.55.0

### Will you send logs?

No