## 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/utils.py
)
from urllib.parse import urljoin, urlparse
import lzstring
import pytz
from dateutil import parser
from dateutil.relativedelta import relativedelta
from django.conf import settings
from django.core.cache import cache
from django.db.models.query import QuerySet
from django.db.utils import DatabaseError
from django.http import HttpRequest, HttpResponse
from django.template.loader import get_template
from django.utils import timezone
from rest_framework.exceptions import APIException
from sentry_sdk import capture_exception, push_scope
from posthog.redis import get_client
from posthog.settings import print_warning
DATERANGE_MAP = {
    "minute": datetime.timedelta(minutes=1),
    "hour": datetime.timedelta(hours=1),
    "day": datetime.timedelta(days=1),
    "week": datetime.timedelta(weeks=1),
    "month": datetime.timedelta(days=31),
}
def format_label_date(date: datetime.datetime, interval: str) -> str:
    labels_format = "%a. {day} %B"
    if interval == "hour" or interval == "minute":
        labels_format += ", %H:%M"
    return date.strftime(labels_format.format(day=date.day))
def absolute_uri(url: Optional[str] = None) -> str: