Fix bad usage of usage message generator

(Irony not intended...)

Error message was passed as usage pattern, ie. calling invalid subcommand
such as:

    mytool nonesuch

would result in usage message like:

    usage:
      mytool invalid sub-command: nonesuch
      mytool [options] COMMAND [ARG...]

    [...]

src/saturnin.sh.skel

     debug -v binpath
     debug "\$*='$*'"
     test -x "$binpath" \
-     || saturnin__usage "invalid sub-command: $subcommand"
+     || saturnin__usage -w "invalid sub-command: $subcommand"
     SATURNIN_SUBCOMMAND="$subcommand" "$binpath" "$@"
 }