Abort when DISPLAY isn't valid

There's not much else to do. There's no return and making the
RenderWindow a zombie object isn't any better. Throwing an exception
isn't good either since nothing else throws exceptions around here.

Change-Id: I4cd36c6ba833f02471764b18e7700796735f6a7a
Merge topic 'fail-gracefully-on-X-errors' into master

1d1fb483 Abort when DISPLAY isn't valid