Message for attempting to enter an occupied boat or minecart or ride an entity already being ridden

[muzikbike]

There's a "This bed is occupied" message which is displayed when trying to enter a bud which is currently in use. A similar message for boats minecarts and mobs which have reached their maximum capacity for riders wouldn't be too out of place, either.

I seem to remember the existence of a mod that allows for mobs in vehicles to be dismounted by right clicking, so it might be worth checking for compatibility issues if this message gets implemented and that mod isn't just a false memory of mine.

Also, this has notified me to a vanilla inconsistency: right clicking on a fully occupied boat plays a hand animation, but the same isn't true of minecarts.