Fix ``KeyError: 'msgtype'``. Use ``.get``
Fix ``KeyError: 'msgtype'``. Use ``.get``

Fixes a key error where the mailer tried to get the ``msgtype`` of an
event that was missing a ``msgtype``.

```
 File "synapse/push/mailer.py", line 264, in get_notif_vars
 File "synapse/push/mailer.py", line 285, in get_message_vars
 File ".../frozendict/__init__.py", line 10, in __getitem__
    return self.__dict[key]
    KeyError: 'msgtype'
```

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