Fix when no expressions or keep_warm is false

When I set keep_warm_expression it fails with 
```
    events=events
  File "/Users/valery/.local/share/virtualenvs/document_data_extract_zappa-j8vc3Vbe/lib/python3.6/site-packages/zappa/core.py", line 2783, in schedule_events
    print("Could not create event {} - Please define either an expression or an event source".format(name))
UnboundLocalError: local variable 'name' referenced before assignment
```

So I fixed it.

zappa/core.py

@@ -2780,6 +2780,7 @@ def schedule_events(self, lambda_arn, lambda_name, events, default=True):
                 elif rule_response == 'dryrun':
                     print("Dryrun for creating {} event schedule for {}!!".format(svc, function))
             else:
+                name = self.get_scheduled_event_name(event, function, lambda_name)
                 print("Could not create event {} - Please define either an expression or an event source".format(name))