OC_Util::setupFS($user) will create a data dir for the given string - no matter if the user really exists - OCP\JSON::checkUserExists($owner); introduces a ready to use check which will bail out with an JSON error
Merge branch 'master' into fix-setupfs-only-for-existing-users-master
Merge pull request #6858 from owncloud/fix-setupfs-only-for-existing-users-master

OC_Util::setupFS($user) will create a data dir for the given string - no...