transaction: downgrade warnings about wanted unit which are not found

transaction: downgrade warnings about wanted unit which are not found

If a unit was pulled by a Wants= dependency but its unit file was not
present then we logged this as an error.

However Wants= might be used to configure a soft/optional dependency
on another unit, ie. start an optional service only if it's installed
otherwise simply skip it. In this case emitting an error doesn't look
appropriate.

But it's still an error if the optional dependency exists but its
activation fails for any reasons.