detect if running without `with_scheduler`

[aDotInTheVoid]

Several times, I've been tricked into thinking there's a bug due to using when while not inside of with_scheduler. We should make a best-effort attempt to detect this.

[aDotInTheVoid]

Short term fix: check some global state when calling when

Long term fix: change cpp verona-rt to expose failure to spawn behaviour nicely

[aDotInTheVoid]

You could also change the API to have some token that indicates the runtime is there, but this is painfull. Tokio just panics