Only show one error toast at a time

[Amnish04]

@tarasglek wanted to only show the last error on screen to avoid confusion with lots of stacked error toasts.

This quick PR does exactly that by keeping track of any open error toast ids, and closing them whenever a new error toast is triggered.

Here's a quick demo of how it works.

It wasn't clear to me if this was exactly what we wanted, or hide any open toasts on submitting a completion, so I went with whatever I understood.

One problem I see with this approach is that some important error messages might be lost if multiple relevant error toasts are relevant at a time.

This fixes #667

[humphd]

I think this is good. We retain the error in the console logs.

[tarasglek]

love it. thank you.