pid1: improve message when setting up namespace fails

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.