Fix error reporting in @stack_delete_after unit tests

If a unit test decorated with @stack_delete_after (to ensure that a stack
being tested is cleaned up) fails in such a way that the stack cannot be
cleaned up, the original error was suppressed, making debugging very
difficult. In this case, we should re-raise the original exception.

Change-Id: I2b0c326f819cd8a6bc772fe52deb14fb1c41bf93