Stop relying on the message text of ZeroDivisionError in tests.

The test for broken_repr relied on the message text of
ZeroDivisionError, which is different on pypy (just "integer
division", not "integer division or modulo"). This causes a test
failure. Fix this by explicitly raising an exception with a message we
provide.
Merge pull request #193 from marienz/tweak-test-for-pypy

Stop relying on the message text of ZeroDivisionError in tests.