Improve the error message on judge not found.

Assets/Cryville/Crtr/Ruleset/Judge.cs

@@ -90,7 +90,10 @@ namespace Cryville.Crtr.Ruleset {
 		}
 		void InsertEvent(Chart.Judge ev, Clip clip, Identifier id, NoteHandler handler, JudgeCallContext call = default(JudgeCallContext)) {
 			if (id.Key == _var_pause) throw new InvalidOperationException("Cannot assign the special judge \"pause\" to notes");
-			var def = _rs.judges[id];
+			JudgeDefinition def;
+			if (!_rs.judges.TryGetValue(id, out def)) {
+				throw new ArgumentException(string.Format("The chart uses a judge named \"{0}\" that is undefined in the ruleset.", id));
+			}
 			_etor.Evaluate(_identop, def.input);
 			_etor.Evaluate(_clipop, def.clip);
 			var list = evs[_identbuf];