[bug 911846] Switch the repr to a unicode

This fixes errors in production kicked up by the error handling
for whatever errors the error handling was handling.

In Django, you define a `__unicode__` and Model handles `__str__` and `__repr__` for you.

* https://docs.djangoproject.com/en/1.4/ref/models/instances/#unicode
* https://github.com/django/django/blob/1.4.6/django/db/models/base.py#L371

There's no test here because this is like defining a class without subclassing object (which makes it a new-style object)--it's not the sort of trivial thing you write tests for.

r?