Avoid using except Exception in unit test

Failed assertions raise AssertionError, which is a subclass of Exception.
Catching Exception and ignoring it then ends up ignoring any failed
exceptions caught by the exception handler.

Using a more specific sentinel exception prevents the ignoring of failed
assertions from happening

Change-Id: I93b3725e16adc16457580d5150f3d88a86d614dd
Closes-Bug: 1378088
Merge "Avoid using except Exception in unit test"