## 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
properties={"$current_url": "https://something.com", "another_key": "value",},
            )
            event_p2 = event_factory(
                team=self.team,
                distinct_id="person2",
                event="$pageview",
                properties={"$current_url": "https://something.com"},
            )
            
            _, _, team2 = Organization.objects.bootstrap(None)
            person_team2 = person_factory(
                team_id=team2.pk, distinct_ids=["person_team_2"], properties={"group": "another group"}
            )
            event_team2 = event_factory(
                team=team2,
                distinct_id="person_team_2",
                event="$pageview",
                properties={"$current_url": "https://something.com", "another_key": "value",},
            )
            filter = Filter(data={"properties": [{"key": "group", "value": "some group", "type": "person"}]})
            events = filter_events(filter=filter, team=self.team, person_query=True, order_by=None)
            self.assertEqual(len(events), 1)
            self.assertEqual(events[0]["id"], event2.pk)
            filter = Filter(
                data={"properties": [{"key": "group", "operator": "is_not", "value": "some group", "type": "person"}]}
            )
            events = filter_events(filter=filter, team=self.team, person_query=True, order_by=None)
            self.assertEqual(events[0]["id"], event_p2.pk)
            self.assertEqual(len(events), 1)
        def test_user_properties_numerical(self):