Provide better errors

[jpnurmi]

A missing route should throw a FlutterError with a clear description instead of a vague null check operator error:

Steps to reproduce:
While testing #656, I changed the initial route but forgot to restore the route itself:

diff --git a/packages/ubuntu_desktop_installer/lib/routes.dart b/packages/ubuntu_desktop_installer/lib/routes.dart
index fcab20a0..7c0a33fc 100644
--- a/packages/ubuntu_desktop_installer/lib/routes.dart
+++ b/packages/ubuntu_desktop_installer/lib/routes.dart
@@ -1,5 +1,5 @@
 abstract class Routes {
-  static const initial = welcome;
+  static const initial = chooseYourLook;
   static const welcome = '/welcome';
   static const tryOrInstall = '/tryorinstall';
   static const turnOffRST = '/turnoffrst';

Imported from #29