## 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
def __init__(self, klass):
        self.klass = klass
        self.instance = None
    def __call__(self, *args, **kwds):
        if self.instance == None:
            self.instance = self.klass(*args, **kwds)
        return self.instance
def get_machine_id() -> str:
    """A MAC address-dependent ID. Useful for PostHog instance analytics."""
    
    
    return hashlib.md5(uuid.getnode().to_bytes(6, "little")).hexdigest()
def get_table_size(table_name):
    from django.db import connection
    query = (
        f'SELECT pg_size_pretty(pg_total_relation_size(relid)) AS "size" '
        f"FROM pg_catalog.pg_statio_user_tables "
        f"WHERE relname = '{table_name}'"
    )
    cursor = connection.cursor()
    cursor.execute(query)
    return dict_from_cursor_fetchall(cursor)
def get_table_approx_count(table_name):