Silent failure to set up key backup download (missing backup decryption key)

### Steps to reproduce

1. Start with a device ("device A") that does not have the key backup decryption key (in practice this might happen because the key is missing from secret storage, or perhaps there some error when setting up key backup in the first place. For testing, you can simulate it in EW by deleting the `recovery_key_v1` key from the `backup_keys` store in the `matrix-js-sdk::matrix-sdk-crypto` indexeddb, via the dev tools.)
2. Log in on a new device ("device B"), and verify the new device against device A.

### Outcome

#### What did you expect?

A fully-functional client with access to encrypted room history

#### What happened instead?

No access to room history: all historical events are UTD

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

_No response_

### Homeserver

_No response_

### Will you send logs?

No