Fix error message when binding files (#7196)

Fix error message when binding files (#7196)

If you use machinectl to bind a file into a container, it responds with a confusing error message about a temporary directory not being a directory.

I just swapped it to error with the source that was passed, rather than the tmpdir.

It would also be nice to be able to bind files, but that's a separate issue (#7195).

Before the change:

root@epona /var/lib/sandbox $ cat bar/foo
Hello world!
root@epona /var/lib/sandbox $ machinectl bind testing /var/lib/sandbox/bar/foo /foo
Failed to bind mount: Failed to overmount /tmp/propagate.W5TNsj/mount: Not a directory

After the change:

root@epona /var/lib/sandbox $ machinectl bind testing /var/lib/sandbox/bar/foo /foo
Failed to bind mount: Failed to overmount /var/lib/sandbox/bar/foo: Not a directory