Report an error when something goes wrong

* If an engine attempts to make an illegal move
* If an engine claims we allowed an illegal move

There ought to also be a descriptive error message, but there isn't yet.
But it's way better than appearing to freeze.

https://bugzilla.gnome.org/show_bug.cgi?id=704986

src/chess-engine-cecp.vala

@@ -66,6 +66,8 @@ public class ChessEngineCECP : ChessEngine
 
             if (line.has_prefix ("Illegal move: "))
             {
+                stop ();
+                error ();
             }
             else if (line == "resign" || line == "tellics resign")
             {

src/chess-engine.vala

@@ -26,6 +26,7 @@ public abstract class ChessEngine : Object
     public signal void moved (string move);
     public signal void resigned ();
     public signal void stopped ();
+    public signal void error ();
     
     private bool _ready = false;
     public bool ready

src/chess-game.vala

@@ -1239,7 +1239,8 @@ public enum ChessResult
     IN_PROGRESS,
     WHITE_WON,
     BLACK_WON,
-    DRAW
+    DRAW,
+    BUG
 }
 
 public enum ChessRule
@@ -1252,7 +1253,8 @@ public enum ChessRule
     INSUFFICIENT_MATERIAL,
     RESIGN,
     ABANDONMENT,
-    DEATH
+    DEATH,
+    BUG
 }
 
 public class ChessGame
@@ -1531,7 +1533,7 @@ public class ChessGame
         }
     }
 
-    private void stop (ChessResult result, ChessRule rule)
+    public void stop (ChessResult result, ChessRule rule)
     {
         this.result = result;
         this.rule = rule;

src/gnome-chess.vala

@@ -516,6 +516,7 @@ public class Application : Gtk.Application
             opponent_engine.ready_changed.disconnect (engine_ready_cb);
             opponent_engine.moved.disconnect (engine_move_cb);
             opponent_engine.stopped.disconnect (engine_stopped_cb);
+            opponent_engine.error.disconnect (engine_error_cb);
             opponent_engine = null;
         }
 
@@ -547,6 +548,7 @@ public class Application : Gtk.Application
             opponent_engine.ready_changed.connect (engine_ready_cb);
             opponent_engine.moved.connect (engine_move_cb);
             opponent_engine.stopped.connect (engine_stopped_cb);
+            opponent_engine.error.connect (engine_error_cb);
             opponent_engine.start ();
         }
 
@@ -656,7 +658,8 @@ public class Application : Gtk.Application
     
     private void engine_move_cb (ChessEngine engine, string move)
     {
-        opponent.move (move);
+        if (!opponent.move (move))
+            game.stop (ChessResult.BUG, ChessRule.BUG);
     }
 
     private void engine_stopped_cb (ChessEngine engine)
@@ -664,6 +667,11 @@ public class Application : Gtk.Application
         opponent.resign ();
     }
 
+    private void engine_error_cb (ChessEngine engine)
+    {
+        game.stop (ChessResult.BUG, ChessRule.BUG);
+    }
+
     private void game_start_cb (ChessGame game)
     {
         if (opponent_engine != null)
@@ -1005,6 +1013,11 @@ public class Application : Gtk.Application
             title = _("Game is drawn");
             pgn_game.result = PGNGame.RESULT_DRAW;            
             break;
+        case ChessResult.BUG:
+            /* Message display when the game cannot continue */
+            title = _("Oops! Something has gone wrong.");
+            /* don't set the pgn_game result; these are standards */
+            break;
         default:
             break;
         }
@@ -1058,6 +1071,11 @@ public class Application : Gtk.Application
             reason = _("One of the players has died");
             pgn_game.termination = PGNGame.TERMINATE_DEATH;
             break;
+        case ChessRule.BUG:
+            /* Message displayed when something goes wrong with the engine */
+            reason = _("The game cannot continue.");
+            /* Don't set pgn_game termination; these are standards*/
+            break;
         }
 
         info_title_label.set_markup ("<big><b>%s</b></big>".printf (title));