sync_mailinglists errors

Ghost User
2019-12-18T19:40:53+01:00

### One-sentence description
The `sync_mailinglists` generates many errors.

### Description

Currently, the `sync_mailinglists` generates many errors (see the [server sentry issues](https://sentry.io/organizations/thalia/issues/?project=1470866)).
These should either be caught or the underlying bug should be fixed.