Prevent non-superscriptable object from exploding

Changing the order of the test for whether or not an error exists stops the
test runner from ending preemptively with a `TypeError` for object types that
are not superscriptable. (For exmple, the `Response` objects that come from
`Flask`). 

This change prevents the `TypeError` when looking for
`test['topic']['error']`, but allows the `VowAssertionError` to bubble up,
including its message about the actual failed assertion while preserving
the related stack trace.