x/website/tour: "Run" button becomes "Kill" on compilation error

[enocom]

To reproduce, visit any page with a playground on https://go.dev/tour:

1. Write something that will not compile, e.g.,

package main

// missing close quote
import "fmt 

func main() {
	fmt.Println("Hello, World")
}

2. Click "Run".
3. Compilation error is reported, but the "Run" button is now stuck on "Kill"

Expected behavior:

After clicking "Run", the button should return to "Run" with a compilation error.

[gopherbot]

Change https://go.dev/cl/478415 mentions this issue: tour: call 'done' callback after Run, on message Kind 'system'