Very generic error when mail server is not configured?

[Terri]

If postfix isn't running correctly, mailman throws an error upon creating a list. Which is fine.

Postorius interprets this as a very generic 500 error at the web interface. I haven't decided if that's fine, or if we should provide a slightly more educational error message. On one hand, "contact your admin/something went wrong" is pretty reasonable for J. Random User, but this particular error is more likely to occur during setup so it might be useful to provide a more admin-friendly message to aid in debugging. This bug report is to remind us to consider the ideal error message here.