I don't think you can raise a string. Maybe in old versions of python you could? Instead of doing this check at all, you could just let the call to ```shutil.copytree()``` raise an exception if the source doesn't exist, which will be an ```OSError```.

In any case, you should be prepared to catch any exceptions raised in this method (probably just ```OSError```) where it gets called, so rather than crapping out with an unhandled exception, the distributor will return a report explaining that it failed and how.