Add test in import code to make sure two overlapping courses aren't imported

We have been having an issue where the course term changes and gets imported via the command line. This completes successfully and creates two courses that have overlapping items in the modulestore. This results in major problems when one course is deleted as it fires errors with findone finding more than one location as well as deleting portions of both courses when one is partially deleted.  

This code corrects this by logging an error when the bad import occurs and skipping the import that would cause the bad modulestore state to occur.  This is already a condition that is checked for in studio, but this enforces it at the import level so that it can't occur via the management command or other possible xml import entry points either.

FYI to @pdpinch and @ichuang

@sarina not sure on reviewers