core/loopback-setup: make log messages nicer

Under nspawn, systemd would print:

Got address error code: Operation not permitted
Got address error code: Operation not permitted
Got start error code: Operation not permitted

which is quite unclear out of context. Change that to:

Failed to add address 127.0.0.1 to loopback interface: Operation not permitted
Failed to add address ::1 to loopback interface: Operation not permitted
Failed to bring loopback interface up: Operation not permitted