Warn user when luigi is not installed, for relevant view

better than getting a server error

tailbone/views/luigi.py

@@ -62,9 +62,22 @@ class LuigiTaskView(MasterView):
     def __init__(self, request, context=None):
         super(LuigiTaskView, self).__init__(request, context=context)
         app = self.get_rattail_app()
+
+        # nb. luigi may not be installed, which (for now) may prevent
+        # us from getting our handler; in which case warn user
+        try:
             self.luigi_handler = app.get_luigi_handler()
+        except Exception as error:
+            self.luigi_handler = None
+            self.luigi_handler_error = error
+            log.warning("could not get luigi handler", exc_info=True)
 
     def index(self):
+
+        if not self.luigi_handler:
+            self.request.session.flash("Could not create handler: {}".format(
+                simple_error(self.luigi_handler_error)), 'error')
+
         luigi_url = self.rattail_config.get('rattail.luigi', 'url')
         history_url = '{}/history'.format(luigi_url.rstrip('/')) if luigi_url else None
         return self.render_to_response('index', {
@@ -147,14 +160,20 @@ class LuigiTaskView(MasterView):
         return context
 
     def get_overnight_tasks(self):
+        if self.luigi_handler:
             tasks = self.luigi_handler.get_all_overnight_tasks()
+        else:
+            tasks = []
         for task in tasks:
             if task['last_date']:
                 task['last_date'] = six.text_type(task['last_date'])
         return tasks
 
     def get_backfill_tasks(self):
+        if self.luigi_handler:
             tasks = self.luigi_handler.get_all_backfill_tasks()
+        else:
+            tasks = []
         for task in tasks:
             if task['last_date']:
                 task['last_date'] = six.text_type(task['last_date'])