response['events_received'] doesn't exist if input invalid (#669)

Summary:
Changed to process error

Pull Request resolved: #669

Reviewed By: liliarizona

Differential Revision: D61728915

Pulled By: stcheng

fbshipit-source-id: e91fd7de789a7f49ced987200e54e6533e1f8f0a

facebook_business/adobjects/serverside/event_request_async.py

@@ -90,9 +90,14 @@ async def create_event(self, session):
     async def execute(self):
         async with aiohttp.ClientSession() as session:
             response = await self.create_event(session)
-            return EventResponse(events_received=response['events_received'],
+            if response.get('events_received'):
+                return EventResponse(events_received=response['events_received'],
                                            fbtrace_id=response['fbtrace_id'],
                                            messages=response['messages'])
+            else 
+                return EventResponse(events_received=0,
+                                           fbtrace_id=response['fbtrace_id'],
+                                           messages=response['messages']) 
 
     def normalize(self):
         normalized_events = []