x11 - cleanup error path and overwrite DISPLAY if X11 forwarding failed.

Set DISPLAY to SLURM_X11_SETUP_FAILED to make it clear that the
tunnel setup has failed. This at least gives the user a hint as to
why their X11 apps aren't working, although further refinement should be
done later:

tim@zoidberg:~$ srun --x11 xclock
Error: Can't open display: SLURM_X11_SETUP_FAILED
srun: error: node001: task 0: Exited with exit code 1