engine: confusing error message retrieving from http session

- Fixed confusing error message when retrieving non existing value
  from http session. Instead of throwing error message in log, just
  return null.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1101551

Change-Id: I336430e35bbbad07ea9566c8f5293980056361de
Signed-off-by: Alexander Wels <awels@redhat.com>