usage help when mwscript is not run as `apache` user

7c4810c8a limited the execution of MWScript to prevent running it as a
privileged user (wikidev, root, sudo groups).  This patch improve the
error message and reference the `sudo -u apache` command that should be
used before the command.  That will help new comers and people not used
to run mwscript commands.

Change-Id: I3aeb6b28382828cb37dac4863cf55602a0235f05