Fix usage when _pretty__* called directly

exit does not work as we want in subshell, so we need to exit higher.

src/include/pretty.sh

         _pretty__is_internal && continue
         _pretty__is_frontend && echos "$fname" && return 0
         echo "do not call _pretty_* directly: $fname" >&2
-        exit_usage
+        return $FFOO_EXIT_USAGE
     done
 }
 
     local frontend  # who (of pretty.sh) was called (=> prettyprinter choice)
     local caller    # which user's function (or script) called it
     local provider  # which provider (_pretty__echo_*()) to use
-    frontend="$(_pretty__get_frontend)"
+    frontend="$(_pretty__get_frontend)" || exit_usage
     caller="$(_pretty__get_caller)"
     test "$caller"   == "main"       && caller="$(basename "$0")"
     test "$caller"   == "usage"      && frontend=mkusage