## 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
try:
        return datetime.datetime.strptime(input, "%Y-%m-%d").replace(tzinfo=pytz.UTC)
    except ValueError:
        pass
    
    
    try:
        return parser.isoparse(input).replace(tzinfo=pytz.UTC)
    except ValueError:
        pass
    regex = r"\-?(?P<number>[0-9]+)?(?P<type>[a-z])(?P<position>Start|End)?"
    match = re.search(regex, input)
    date = timezone.now()
    if not match:
        return date
    if match.group("type") == "h":
        date = date - relativedelta(hours=int(match.group("number")))
        return date.replace(minute=0, second=0, microsecond=0)
    elif match.group("type") == "d":
        if match.group("number"):
            date = date - relativedelta(days=int(match.group("number")))
    elif match.group("type") == "m":
        if match.group("number"):
            date = date - relativedelta(months=int(match.group("number")))
        if match.group("position") == "Start":