In multiworker mode better error message if no apps

[ahopkins]

If a developer does not define their app so that a worker process can access it, they will receive a KeyError when Sanic tries to access it from the registry.

For example, that would happen if an app only was instantiated on the main process:

if __name__ == "__main__":
    app = Sanic(...)
    app.run()

The solution to this is usually either to move the instantiation outside that block, or to use an AppLoader.

We should provide a better error message from within a worker process if an app cannot be found in a registry.

[scardozos]

I want to take it🙂

[prryplatypus]

All yours 😛