Fix map importer

OpenRA.Game/ActorReference.cs

@@ -10,6 +10,7 @@
 
 using System.Collections.Generic;
 using System.Collections;
+using System.IO;
 
 namespace OpenRA.FileFormats
 {
@@ -22,6 +23,9 @@ namespace OpenRA.FileFormats
 
 		public ActorReference( string type, Dictionary<string, MiniYaml> inits )
 		{
+			if (!Rules.Info.ContainsKey(type))
+				throw new InvalidDataException("Unknown actor: `{0}'".F(type));
+			
 			Type = type;
 			InitDict = new TypeDictionary();
 			foreach( var i in inits )