Fix TypeError for class_mapper called w/ iterable

When the `class_` passed is not a mapped class but is actually an iterable, the string formatting operation fails with a `TypeError: not all arguments converted` exception, and the expected `ArgumentError` is not raised. Calling code which is using reflection and expects this error will fail (e.g. the [sadisplay](https://pypi.python.org/pypi/sadisplay "sadisplay : Python Package Index") module).