core/load-droping: avoid oom warning when the unit symlink is not a template

unit_name_template returns -EINVAL if the unit name is not a template, but
the code assumed that OOM is the only failure mode. Fix that to emit the warning
if a non-template unit is encountered (because in this case we expect the name
to match exactly), and just skip the warning on other errors (presumably oom).

Fixes #5543.