log unhandled celery exceptions

they were always caught locally by celery's base handler, however,
we weren't logging them ourselves, which meant it wouldn't be put into
the json logs that are sent to cloudwatch.

app/celery/celery.py

@@ -1,17 +1,28 @@
-from celery import Celery
+from flask import current_app
+from celery import Celery, Task
+
+
+class NotifyTask(Task):
+    abstract = True
+
+    def on_failure(self, exc, task_id, args, kwargs, einfo):
+        # ensure task will log exceptions to correct handlers
+        current_app.logger.exception('Celery task failed')
+        super().on_failure(exc, task_id, args, kwargs, einfo)
+
+    def __call__(self, *args, **kwargs):
+        # ensure task has flask context to access config, logger, etc
+        with current_app.app_context():
+            return super().__call__(*args, **kwargs)
 
 
 class NotifyCelery(Celery):
 
     def init_app(self, app):
-        super().__init__(app.import_name, broker=app.config['BROKER_URL'])
+        super().__init__(
+            app.import_name,
+            broker=app.config['BROKER_URL'],
+            task_cls=NotifyTask,
+        )
+
         self.conf.update(app.config)
-        TaskBase = self.Task
-
-        class ContextTask(TaskBase):
-            abstract = True
-
-            def __call__(self, *args, **kwargs):
-                with app.app_context():
-                    return TaskBase.__call__(self, *args, **kwargs)
-        self.Task = ContextTask