## Bug description

The following filter fails with a 500 internal error.

![2021-02-04_12-07](https://user-images.githubusercontent.com/148820/106877644-a9f4ce80-66e1-11eb-8320-87bb213bdde2.png)


## Expected behavior

Error is shown to user about invalid regular expression.

Note no errors shown on the page as well.

## How to reproduce

1. Go to events
2. Add filter "Current URL matches regular expression ?*"
3. See infinite loading screen / error

## Environment

- [x] PostHog Cloud
- [x] self-hosted PostHog, version/commit: _please provide_

## Additional context

Note this can also happen when filtering by events.

#### *Thank you* for your bug report – we love squashing them!

posthog/models/filters/test/test_filter.py
event_factory(team=self.team, distinct_id="test", event="$pageview")
            event2 = event_factory(
                team=self.team, event="$pageview", distinct_id="test", properties={"is_first": True},
            )
            filter = Filter(data={"properties": {"is_first": "true"}})
            events = filter_events(filter, self.team)
            self.assertEqual(events[0]["id"], event2.pk)
        def test_is_not_true_false(self):
            event = event_factory(team=self.team, distinct_id="test", event="$pageview")
            event2 = event_factory(
                team=self.team, event="$pageview", distinct_id="test", properties={"is_first": True},
            )
            filter = Filter(data={"properties": [{"key": "is_first", "value": "true", "operator": "is_not"}]})
            events = filter_events(filter, self.team)
            self.assertEqual(events[0]["id"], event.pk)
        def test_json_object(self):