[UX] Gracefully stop the Board run

[donsbach]

When i stop the board the Activities panel looks like it's still running and the board errors out. Can we rather just stop the animation and show a message that says, "The board has been stopped."

[dglazkov]

@paullewis would it help if I plumb "abort" as a distinct signal from the runner?

[paullewis]

The current implementation is that I call abort on the AbortSignal, but I also have to clear out any pending inputs, and that gives rise to this case because the final input (which is usually fulfilled as an empty value) causes these kinds of outputs.

I suspect we need to emit a final event from the runner when we abort that closes off any open events and issues a nicer looking message?