Error checking inputs and reverting if illegal

app/src/main/java/gui/SudokuController.java

@@ -127,6 +127,15 @@ public class SudokuController {
 
             // Update the model and view
             model.set(row, col, value);
+
+            // Warn if the board is not solvable (e.g. if the user has made a mistake)
+            // This is very messy, error prone and computationally expensive
+            if(!model.isSolvable()) {
+                model.set(row, col, 0);
+                view.showErrorMessage("Illegal move. The board is not solvable.");
+            }
+
+            // Sync the view with the model
             view.updateView(model.getBoard());
         }
     }