Fix churn crash on no source control system found

This deals with the case where no source control system is found for churn. It requires any of a few different source control systems to be available to check on the churn. This change makes it output the error to the console when metric fu is run, but it doesn't crash metric fu. It  will detect that it didn't collect the churn output and just have a empty churn report for the project.

resolves: https://github.com/metricfu/metric_fu/issues/65