Failures to deserialize/decrypt backed-up room keys are not shown in the UI

Suppose we find a message which was sent after we registered, but before we logged in. We attempt to download the keys for such messages from the server-side store.

If the key is not found, there is obviously little we can do. However, if we find a key but then cannot decrypt it, or deserialize it, we should probably tell the user that there was an error -- or at least, stop showing "waiting for this message".

This particularly affects users who had old versions of Fluffychat, due to https://github.com/famedly/matrix-dart-sdk/issues/1739 -- but there may be other circumstances in which the key is in some way corrupt.

Start by tracing the backed-up room-key download path, including where returned keys are deserialized and decrypted. Verify how failures are propagated to the message state and UI; done means corrupt or undecryptable keys produce an error indication instead of leaving messages showing "waiting for this message".

Written by the indexing model from the issue text.