## 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
event1 = event_factory(team=self.team, event="$pageview", distinct_id="test",)
            event2 = event_factory(
                team=self.team,
                event="$pageview",
                distinct_id="test",
                properties={"$current_url": "https://something.com"},
            )
            event_factory(
                team=self.team,
                event="$pageview",
                distinct_id="test",
                properties={"$current_url": "https://whatever.com"},
            )
            event3 = event_factory(
                team=self.team, event="$pageview", distinct_id="test", properties={"$current_url": None},
            )
            filter = Filter(data={"properties": {"$current_url__not_icontains": "whatever.com"}})
            events = filter_events(filter, self.team, order_by="id")
            self.assertEqual(sorted([event["id"] for event in events]), sorted([event1.pk, event2.pk, event3.pk]))
            self.assertEqual(len(events), 3)
        def test_multiple(self):