show an error message if project cannot be loaded (fix #190)

gui.js

@@ -284,7 +284,8 @@ IDE_Morph.prototype.openIn = function (world) {
             }
             throw new Error('unable to retrieve ' + url);
         } catch (err) {
-            return;
+            myself.showMessage('unable to retrieve project');
+            return '';
         }
     }