sysusers: emit a bit more info at debug level when locking fails

This is the first error message when running unprivileged, and the message is
unspecific, so let's at least add some logging at debug level to make this less
confusing.

src/sysusers/sysusers.c

@@ -1841,7 +1841,7 @@ int main(int argc, char *argv[]) {
 
         lock = take_etc_passwd_lock(arg_root);
         if (lock < 0) {
-                log_error_errno(lock, "Failed to take lock: %m");
+                log_error_errno(lock, "Failed to take /etc/passwd lock: %m");
                 goto finish;
         }