journalctl: various fixes to the access check logic

- Reword messages a bit

- Correct check whether EACCES is in the set of errors

- Don't complain if no journal files are found

- allocate Set object for errors lazily since in the best case we don't
  need it at all.

- don't consider it an error if /run/log/journal doesn't exist (because
  that's the usual case actually, if storage is enabled)