## 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
"""
    Returns an absolutely-formatted URL based on the `SITE_URL` config.
    """
    if not url:
        return settings.SITE_URL
    return urljoin(settings.SITE_URL.rstrip("/") + "/", url.lstrip("/"))
def get_previous_week(at: Optional[datetime.datetime] = None) -> Tuple[datetime.datetime, datetime.datetime]:
    """
    Returns a pair of datetimes, representing the start and end of the week preceding to the passed date's week.
    """
    if not at:
        at = timezone.now()
    period_end: datetime.datetime = datetime.datetime.combine(
        at - datetime.timedelta(timezone.now().weekday() + 1), datetime.time.max, tzinfo=pytz.UTC,
    )  
    period_start: datetime.datetime = datetime.datetime.combine(
        period_end - datetime.timedelta(6), datetime.time.min, tzinfo=pytz.UTC,
    )  
    return (period_start, period_end)
def exception_reporting(exception: BaseException, context: Dict) -> None:
    """
    Determines which exceptions to report to Sentry and sends them.
    """
    if not isinstance(exception, APIException):
        capture_exception(exception)
def relative_date_parse(input: str) -> datetime.datetime: