[story_shell] show snack bar on error

Adds a snackbar to the module when adding
the module fails.

Change-Id: I05d1f436b38716898515484defb71008896a7853
diff --git a/lib/story_shell/examples/example_manual_relationships/lib/start_module_button.dart b/lib/story_shell/examples/example_manual_relationships/lib/start_module_button.dart
index bc43b40..bc45756 100644
--- a/lib/story_shell/examples/example_manual_relationships/lib/start_module_button.dart
+++ b/lib/story_shell/examples/example_manual_relationships/lib/start_module_button.dart
@@ -35,6 +35,8 @@
         onPressed: () {
           log.fine('starting module with relation $_relation');
           _startChildModule(_relation).catchError((e) {
+            Scaffold.of(context).showSnackBar(
+                SnackBar(content: Text('Failed to start Module, see syslog')));
             log.warning('Failed to start child module', e);
           });
         },