[SPARK-5161] [HOTFIX] Fix bug in Python test failure reporting

This patch fixes a bug introduced in #7031 which can cause Jenkins to incorrectly report a build with failed Python tests as passing if an error occurred while printing the test failure message.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7112 from JoshRosen/python-tests-hotfix and squashes the following commits:

c3f2961 [Josh Rosen] Hotfix for bug in Python test failure reporting