Don't try to mail errors when missing config.

Avoid a bogus log entry like:

```
INFO: Sending warning email to None
DEBUG: Emailing:
-------------
To: (None,)
From: luigi-client@Joes-MacBook-Pro.local
... lots more debug logging ...
```

Which now becomes:
```INFO: Skipping error email. Set `error-email` in the `core` section of the luigi config file to receive error emails.```