transaction.c: do not point users to logs when unit not found

The logs are unlikely to contain any useful information in this case.

Also, change "walked on cycle path" to "found dependency on", which
is less technical and indicates the direction. With the old message,
I was never sure if prior units depended on later ones, or vice versa.

https://bugzilla.redhat.com/show_bug.cgi?id=996133
https://bugzilla.redhat.com/show_bug.cgi?id=997082