Fix session locking once again.

A logic bug was introduced by 61fd1270da44aebb1a4fe1a1ce4a4292b99e5545.
That is, if the locking was a dry-run, overall status is indicated by
just "success" field which reports about D-Bus lock status, and
lock-file status shall not be taken into account (because on a real
lock, if D-Bus name is free and lock-file is taken, the latter is
force-removed).

But for non-dry-run locks, that commit was a mistake: for them,
if a lock-file could not be taken even after force-removal,
it will be a locking failure.