Description
If you try to route a message to a non-existent transport/sender, the message is not very clear:

The service "messenger.middleware.send_message" has a dependency on a non-existent service "async".

We need to catch this earlier, probably in a compiler pass. It could say:

The App\Message\SmsNotification class is being routed to a sender called async. This is not a valid transport or service id.