Fixed with fstab.
I experienced the same stop job error issue mounting a samba share on a rpi3 running the latest osmc. I confirm the error was observed with wifi, not wired.
There was a similar Debian bug reported which provided some suggestions.
The following fstab entry resolved the error for me.
//192.168.0.1/SMBDrive/Media /mnt/media cifs sec=ntlm,rw,defaults,guest,uid=1000,iocharset=utf8,x-systemd.requires=network.target,x-systemd.automount,noauto 0 0
I am still testing but the system has shutdown cleanly without error through approximately 10 reboots.