code: def _err(self, msg, err=True):
          
          severity = "FATAL" if err else "NOTE"
          txt = "".join([self.objdoc.doc,
                         "User's command-line:\n\n",
                         "  % go_plot.py {ARGS}\n\n".format(ARGS=" ".join(sys.argv[1:])),
                         "**{SEV}: {MSG}\n".format(SEV=severity, MSG=msg)])
          if err:
              raise RuntimeError(txt)
          sys.stdout.write(txt)
          sys.exit(0)
docs: Print useage and error before exiting.