Exporting Chat messages should tell me how many messages failed to export

### Your use case

#### What would you like to do? && Why would you like to do it?

I would like to export my chat messages as json of an encrypted room. Currently, element-web just exports:

```json
{
"type": "m.room.message",
"room_id": "redacted",
"sender": "redacted",
"content": {
"msgtype": "m.bad.encrypted",
"body": "** Unable to decrypt: DecryptionError: The sender's device has not sent us the keys for this message. **"
},
"origin_server_ts": 1608140190518,
"unsigned": {
"age": 71355218926
},
"event_id": "$redacted",
"user_id": "redacted",
"age": 71355218926
}
```

This however isn't told about anywhere in the UI. This means people that use this feature as a backup for their messages might end up realizing later that half of the export is actually broken and utterly useless. It would be nice to have a number displayed of failed to export messages. As in `X of Y messages failed to export`. This would prevent false trust into this export.

#### How would you like to achieve it?

As said above, a simple mention that some failed would be nice.

### Have you considered any alternatives?

I don't think there are any viable alternatives. Telling the person upfront about issues is a simple and obvious way of communicating this. The UI should be simple on this. Especially considering EMS sells this as a product as part of their Audit Bot, seemingly if the marketing side is to trust.

### Additional context

_No response_