Added notation for wrong markers

Cell.cs

@@ -24,6 +24,11 @@
         public void setMine() { isMine = true; }
         public void mark() { isMarked = true; }
         public void unmark() { isMarked = false; }
+        public void setWrong()
+        {
+            isMarked = false;
+            value = "!";
+        }
         public bool isEmpty() { return value == " "; }
     }
 }

Field.cs

@@ -160,6 +160,8 @@
             {
                 if (!opened.Contains(cells[i]))
                     opened.Add(cells[i]);
+                if (cells[i].isMarked && !cells[i].isMine)
+                    cells[i].setWrong();
             }
         }
         public void mark(int y, int x) { cells[y * width + x].mark(); }