pid1: improve message when setting up namespace fails

I covered the most obvious paths: those where there's a clear problem
with a path specified by the user.

Prints something like this (at error level):
May 21 20:00:01.040418 systemd[125871]: bad-workdir.service: Failed to set up mount namespacing: /run/systemd/unit-root/etc/tomcat9/Catalina: No such file or directory
May 21 20:00:01.040456 systemd[125871]: bad-workdir.service: Failed at step NAMESPACE spawning /bin/true: No such file or directory

Fixes #10972.

src/test/test-ns.c

@@ -75,7 +75,8 @@ int main(int argc, char *argv[]) {
                             PROTECT_HOME_NO,
                             PROTECT_SYSTEM_NO,
                             0,
-                            0);
+                            0,
+                            NULL);
         if (r < 0) {
                 log_error_errno(r, "Failed to setup namespace: %m");