Element-R: withheld messages are no longer shown as such

[richvdh]

If the sender chooses to only send e2e messages to verified devices, then unverified devices will receive a notification of a withheld key. (Specifically, a to-device message of type m.room_key.withheld with a code of m.unverified or m.blacklisted.)

We should reflect these notifications in the UI, rather than just showing a generic "Unable to decrypt message" error. (This used to work with legacy crypto, but has not been re-implemented for Rust Crypto).

We should also give them a different error code in the Posthog analytics, so that they can be treated as "expected UTDs".

[andybalaam]

Added to top of backlog because:

• it is a quick win
• it is UTD+Trust+WebR