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

Conflicts:
	lib/public/json.php
Merge pull request #6869 from owncloud/fix-setupfs-only-for-existing-users-stable6

Fix setupfs only for existing users stable6