## 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
events = Event.objects
    if person_query:
        events = events.add_person_id(team.pk)
    events = events.filter(properties_to_Q(filter.properties, team_id=team.pk))
    if order_by:
        events = events.order_by(order_by)
    return events.values()
class TestDjangoPropertiesToQ(property_to_Q_test_factory(_filter_events, Event.objects.create, Person.objects.create)):  
    def test_person_cohort_properties(self):
        person1_distinct_id = "person1"
        person1 = Person.objects.create(
            team=self.team, distinct_ids=[person1_distinct_id], properties={"$some_prop": 1}
        )
        cohort1 = Cohort.objects.create(team=self.team, groups=[{"properties": {"$some_prop": 1}}], name="cohort1")
        cohort1.people.add(person1)
        filter = Filter(data={"properties": [{"key": "id", "value": cohort1.pk, "type": "cohort"}],})
        matched_person = (
            Person.objects.filter(team_id=self.team.pk, persondistinctid__distinct_id=person1_distinct_id)
            .filter(properties_to_Q(filter.properties, team_id=self.team.pk, is_person_query=True))
            .exists()
        )
        self.assertTrue(matched_person)
class TestDateFilterQ(BaseTest):