## 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
person1 = person_factory(
                team_id=self.team.pk,
                distinct_ids=["person1"],
                properties={"name": {"first_name": "Mary", "last_name": "Smith"}},
            )
            event1 = event_factory(
                team=self.team,
                distinct_id="person1",
                event="$pageview",
                properties={"$current_url": "https://something.com"},
            )
            filter = Filter(
                data={
                    "properties": [
                        {
                            "key": "name",
                            "value": json.dumps({"first_name": "Mary", "last_name": "Smith"}),
                            "type": "person",
                        }
                    ]
                }
            )
            events = filter_events(filter=filter, team=self.team, person_query=True, order_by=None)
            self.assertEqual(events[0]["id"], event1.pk)
            self.assertEqual(len(events), 1)
    return TestPropertiesToQ
def _filter_events(filter: Filter, team: Team, person_query: Optional[bool] = False, order_by: Optional[str] = None):