Exit without tracebacks

[darrenburns]

As a developer, there are situations where I want to be able to exit the app and display a simple message to the end user.

Right now, Textual apps don't immediately exist when asked to, meaning invalid state can cause tracebacks to accumulate inside the App._exit_renderables.

I'd like to control exactly what users see on exit (one specific renderable), rather than the a renderable followed by a bunch of tracebacks.