systemctl: fix silent failure when --root is not found

Some calls to lookup_path_init() were not followed by any log emission.
E.g.:
$ SYSTEMD_LOG_LEVEL=debug systemctl --root=/missing enable unit; echo $?
1

Let's add a helper function and use it in various places.

$ SYSTEMD_LOG_LEVEL=debug build/systemctl --root=/missing enable unit; echo $?
Failed to initialize unit search paths for root directory /missing: No such file or directory
1
$ SYSTEMCTL_SKIP_SYSV=1 build/systemctl --root=/missing enable unit; echo $?
Failed to initialize unit search paths for root directory /missing: No such file or directory
Failed to enable: No such file or directory.
1

The repeated error in the second case is not very nice, but this is a niche
case and I don't think it's worth the trouble to trying to avoid it.

src/systemctl/systemctl-enable.c

@@ -141,7 +141,7 @@ int verb_enable(int argc, char *argv[], void *userdata) {
                 if (STR_IN_SET(verb, "mask", "unmask")) {
                         _cleanup_(lookup_paths_free) LookupPaths lp = {};
 
-                        r = lookup_paths_init(&lp, arg_scope, 0, arg_root);
+                        r = lookup_paths_init_or_warn(&lp, arg_scope, 0, arg_root);
                         if (r < 0)
                                 return r;