The usual way of handling errors in clean() is to raise forms.ValidationError with an appropriate message, and the form will be redisplayed with that message and the user can try again.
But what if an error is detected in clean() that the user cannot recover from, for example the database link goes down or a critical system file goes missing. In that case I don’t want to simply redisplay the form because there’s nothing the user can do; if they submit the form again they’ll very likely just encounter the same error.
Redirecting to an error page would be a better way to handle this kind of situation. Can I do that within clean()?
No, because that’s not where the logic belongs – that logic probably belongs in your view, if you do want to specify that yourself.
In practice, this sort of thing will probably cause your code to emit an error 500, which is what you want. Just make sure your error 500 page is helpfully put together.