Fixes #1909 -  NServiceBus fails to shut down if `IWantToRunWhenBusStartsAndStops.Start` throws

Starts a task that will perform a Parallel.ForEach of IWantToRunWhenBusStartsAndStops.Start.
Parallel.ForEach will block until all iterations are completed or one of them fails. The continuation ensures the exception is observed and marked as handled.