Added a try except to handle unicode in domain lookup. Solves FB:120611

Warns the user of an invalid domain name if unicode is passed into form instead of returning a 500 error.
Solves http://manage.dimagi.com/default.asp?120611
I'm struggling to understand why this was necessary. It seems like it's probably a bug in `Domain.get_by_name` that cannot handle a unicode domain name (if the name does not match any domain then it should return `None` or do whatever is done when a non-matching name is passed in there). I would expect Django to always give you unicode strings in `self.cleaned_data`, so I doubt the problem is there. In any case, this seems like the wrong way to fix this issue, even if domain names cannot contain unicode characters.