Clean up EmailTemplateException

While reviewing #184 I notice that `EmailTemplateException` is defined in models.py and raised there. I introduced this pattern in c39b287fccdf702e0126179360a1b297ae1c1a95 and it should be refactored.

`EmailTemplateException` is the only exception defined in models.py, and it's only raised when a DoesNotExist exception is encountered. Since that's already an appropriate model-layer exception, this logic should be moved out to the consumers. Put another way, the model should not be concerned with setup at this level.