core: improve error message when setting up service mounts
    
    Right now we include the private working directory when we say some files
    where not found, which is confusing. Strip it from the error string.
    
    For example, with a BindPaths=/var/bar that does not exist on the host:
    
    Before:
    
      foo.service: Failed to set up mount namespacing: /run/systemd/unit-root/var/bar: No such file or directory
    
    After:
    
      foo.service: Failed to set up mount namespacing: /var/bar: No such file or directory