Confusing error "The sender has blocked you from receiving this message"

### Steps to reproduce

Reporting on behalf of another user in the community.

* User has a mobile session working fine and a DM with another user. Both see their messages.
* User signs into Element Desktop but doesn't remember their security passphrase, and thus cannot access their encryption keys.
* Secure backup is confirmed to be not set up in either session
* "only send to verified devices" is confirmed off in their settings, both sessions

### Outcome

#### What did you expect?

An error message indicating that messages cannot be decrypted due to missing keys.

#### What happened instead?

User sees "The sender has blocked you from receiving this message" which is not true as the sender has not blocked delivering keys to them, and as a non-technical user they thought the other user had blocked them (knowing it not to be true).

https://github.com/matrix-org/matrix-react-sdk/pull/10202 looks like the pr that changed this message.

Additionally, the top of the screen shows a message saying:

> Reset your keys to prevent future decryption errors

This is also a bit misleading. The user never set up secure backup in the first place it seems. Would expect it to just say "Set up secure backup".

Rageshake logs: https://github.com/matrix-org/element-web-rageshakes/issues/20438

### Operating system

Windows

### Application version

1.11.24

### How did you install the app?

_No response_

### Homeserver

_No response_

### Will you send logs?

Yes

Start by reviewing matrix-react-sdk PR 10202 and the reported rageshake context, then reproduce the missing-encryption-keys scenario described here. Done means the error explains that messages cannot be decrypted and the guidance distinguishes setting up secure backup from resetting existing keys.

Written by the indexing model from the issue text.