get_thumbnail_url crashes when image is missing

Thalia Technicie
2019-10-05T13:06:41+02:00

Sentry Issue: [CONCREXIT-Q](https://sentry.io/organizations/thalia/issues/1204599417/?referrer=gitlab_integration)

```
FileNotFoundError: [Errno 2] No such file or directory: '/concrexit/media/public/avatars/bestuursfoto.jpg'
(26 additional frame(s) were not displayed)
...
File "django/template/library.py", line 214, in render
_dict = self.func(*resolved_args, **resolved_kwargs)
File "activemembers/templatetags/activemembers_cards.py", line 47, in membergroup_member_card
ribbon=ribbon
File "members/templatetags/member_card.py", line 22, in member_card
settings.THUMBNAIL_SIZES['medium'])
File "utils/media/services.py", line 90, in get_thumbnail_url
os.path.getmtime(full_original_path)
File "genericpath.py", line 55, in getmtime
return os.stat(filename).st_mtime
```