Fail build if removing docker images fails in clean-test.

We've been getting into losing streaks where some of the tests consistently
fail in automation until they get run on a new Jenkins slave. Anu noticed that
the clean-test step was failing to remove docker images; this should be an
error in automation. We should never run the product tests with images that we
created on a previous run.

Author: Eric Diven

Makefile

@@ -56,7 +56,7 @@ clean-test:
 	rm -f .coverage
 	rm -fr htmlcov/
 	rm -fr tmp
-	-for image in $$(docker images | awk '/teradatalabs\/pa_test/ {print $$1}'); do docker rmi -f $$image ; done
+	for image in $$(docker images | awk '/teradatalabs\/pa_test/ {print $$1}'); do docker rmi -f $$image ; done
 	@echo "\n\tYou can kill running containers that caused errors removing images by running \`make clean-test-containers'\n"
 
 clean-docs: