Refactor error/exception/message handling

This adds:
- LuaCsSetup.ExceptionHandler so we can decide how we want to handle
exceptions for unit tests
- LuaCsSetup.MessageHandler so we can redirect logs to the XUnit output
  helper

Barotrauma/BarotraumaClient/ClientSource/GameMain.cs

@@ -20,7 +20,7 @@ namespace Barotrauma
 {
     class GameMain : Game
     {
-        public static LuaCsSetup LuaCs;
+        internal static LuaCsSetup LuaCs;
 
         public static bool ShowFPS = false;
         public static bool ShowPerf = false;