## 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 the symbolic name of the current active branch. Will return None in case of failure.
    Example: get_git_branch()
        => "master"
    """
    try:
        return (
            subprocess.check_output(["git", "rev-parse", "--symbolic-full-name", "--abbrev-ref", "HEAD"])
            .decode("utf-8")
            .strip()
        )
    except Exception:
        return None
def get_git_commit() -> Optional[str]:
    """
    Returns the short hash of the last commit.
    Example: get_git_commit()
        => "4ff54c8d"
    """
    try:
        return subprocess.check_output(["git", "rev-parse", "--short", "HEAD"]).decode("utf-8").strip()
    except Exception:
        return None
def render_template(template_name: str, request: HttpRequest, context: Dict = {}) -> HttpResponse: