UTD messages are not updated when we receive a withheld notice

[richvdh]

If we get a message that we can't decrypt, we check if we have previously received an m.room_key.withheld message about that session, and if so, show a different error message.

However, AFAICT, if the m.room_key.withheld arrives after the room message, we don't go back and update the error message.

This is untested: we should check which platforms it applies on.