## 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 django.db import connection
    query = f"SELECT reltuples::BIGINT as \"approx_count\" FROM pg_class WHERE relname = '{table_name}'"
    cursor = connection.cursor()
    cursor.execute(query)
    return dict_from_cursor_fetchall(cursor)
def is_postgres_alive() -> bool:
    from posthog.models import User
    try:
        User.objects.count()
        return True
    except DatabaseError:
        return False
def is_redis_alive() -> bool:
    try:
        get_redis_info()
        return True
    except BaseException:
        return False
def is_celery_alive() -> bool:
    try:
        return get_celery_heartbeat() != "offline"
    except BaseException:
        return False
def is_plugin_server_alive() -> bool: