Handle new moderation error: `user_cannot_be_moderator`

The roomserver now enforces that only registered participants can be assigned the moderator role using the UpdateRole command. To handle cases where the specified user is a guest or (registered) call-in user, we added a new error event to the moderation module. This event needs to be handled by the frontend. The new error looks like this:

{
  "user_cannot_be_moderator"
}