udevadm trigger: log errors and return first failure

When udevadm trigger is called, the list of devices to trigger is always
generated through enumeration, and devices can come and go, so we should not
treat -ENOENT as a failure. But other types of failure should be logged.
It seems they were logged until baa30fbc.

Also, return the first error. (I'm not sure if there are other failure modes
which we want to ignore. If they are, they'll need to be whitelisted like
-ENOENT.).