Add functionality specific logging to validate_bookstore

[mpacer]

In #159 we realized that there is utility in logging more information at the point of validating the bookstore settings. To keep the PR more tightly scoped we only added it to the file system cloning validation.

We need to make a decision on if we want to add logs for:

• overall bookstore validation
• archive validation
• publishing validation
• s3 cloning validation

[willingc]

Nicely written @mpacer. To me, addition of the logs on the others depends on whether this information would be useful for monitoring or debugging. If the admin or developer would benefit, please feel free to add.