don't hide other test results when there are behave failures

The calls to `sys.exit` were left over from a copy-pasted code block from the `behave.__main__`

However, the django test runner runs both standard django unit tests as well as the behave features.

Before this commit, if there were behave failures, we didn't get the standard test result output, and thus if there were unit test failures, we weren't informed about them either.