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

- L10N now converted to string to make them work with json_encode
- Added specific error message when server doesn't allow fopen on URLs
- Fixed client side to correctly show error message in a notification
- Added OCP\JSON::encode() method to encode JSON with support for the
  OC_L10N_String values

And greeting from the Atlanta airport. ;-)